HPL's Commonplace Book has been posted on-line:
[www.wired.com]

This might be as good place as any to post the notes I did a while ago on the dating of the commonplace book entries (Lovecraft's own dates aren't that trustworthy). They are by no means exhaustive -- I only needed to roughly date certain portions for a more general HPL essay I was working on -- but perhaps of interest; at least if one doesn't have Schultz's two vol. edition of the CB (which is very much out of print). In fact the notes don't concern just the dating.
RK = Rheinhart Kleiner; AG = Alfred Galpin (See published letter collections to these correspondence [Hippocampus Press])
FF = O Fortunate Floridian (Univ. of Tampa Press)
SL = HPL, Selected Letters (Arkham House)
AMS = autograph manuscript

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The first mention of the CB is found in a letter to RK of Jan. 23, 1920 (RK.178, SL1.106); another letter of Feb. 10 (RK.180-81, SL1.107-8) cites three early entries (#21, 8, 13).

#6: [derived from Dunsany’s “Idle Days on the Yann” -- probably read in a Dunsany collection in early 1920]
#9: Dr. Eben Spencer plot [cf. HPL to the Gallomo, c. April 1920 (AG.71-73, SL1.100-102), and HPL to RK, January 23, 1920 (RK.180)]
#19: Revise 1907 tale—painting of ultimate horror. [i.e. the non-extant juvenile story “The Picture”]
#24: [derived from Dunsany: “A shop in Go-By Street”]
#25: [the Cthulhu bas-relief dream, cf. HPL to the Gallomo, c. April 1920 (AG.87-88) and HPL to RK, May 21, 1920 (RK.188-89, SL1.114-15)]
#26: [the dream of ancient castle and foe with no head, cf. HPL to the Gallomo, c. April 1920 (AG.86-87) and HPL to RK, May 21, 1920 (RK.188, SL1.114)]
#27: Life and Death [claimed to have been located by George Wetzel, but probably does not exist as a story]
#28: The Cats of Ulthar [written June 15, 1920 (date on AMS fair copy given to RK > Grill-Binkin collection > Book Sail (?) > L.W.Currey > ?), cf. HPL to RK, May 21, 1920 (RK.190-91, SL1.116-17)]
#29: [the dream of the destruction of Providence, cf. HPL to RK, May 21, 1920 (RK.188, SL1.113-14)]
#34-36: [possibly influenced by Hugh Elliot’s Modern Science and Materialism (read before June 1921, cf. SL1.134); #35 incorporated into “From Beyond” (written Nov. 16, 1920)]
#47: [description of Irem from Enc. Britannica, incorporated into “The Nameless City” (written late Jan. 1921, cf. SL1.122)
#49: AZATHOTH—hideous name [c. early 1921, cf. #47 -- Barlow claimed to be a dream-name, like Nyarlathotep, cf. his "Memories of HPL (1934)"]
#55: Man followed by invisible thing. [derived from Bierce’s “The Damned Thing”?]
#56: Book or MS. too horrible to be read [. . .] Haverhill incident. [evidently derived from Bierce’s “Suitable Surroundings” (In the Midst of Life) / probably c. August 1921, i.e., the time of the second Haverhill visit -- something told by C. W. "Tryout" Smith (cf. AG.100)]
#61: A terrible pilgrimage to seek the knighted throne of the far daemon-sultan Azathoth. [perhaps c. October 1921? (the novel fragment written in early June, 1922)]
#80: Shapeless living thing forming nucleus of ancient building. [“The House of the Worm” / “The Shunned House”?]
#81: Marblehead—dream—burying hill—evening—unreality. [HPL’s first Marblehead visit was on Dec. 17, 1922]
#88: [transference of personality to a cat; contemplated for use by Whitehead, cf. FF.65]
#95: Verse ‘The House’ as basis of story. [“The Shunned House”?]
#99: Salem story—the cottage of an aged witch—wherein after her death are found sundry terrible things. [perhaps derived from the Rebekah Nurse house seen on the April (13) 1923 visit to Salem (cf. AG.141ff, SL1.221)—earlier trips to Salem in December 1922 (RK.226, SL1.203-204) and February 1923 (AG.128, SL1.213)]
#100-101: [probably incorporated into “The Festival”]
#107: Wall paper cracks off in sinister shape [according to HPL, lead into “The Rats in the Walls” (written c. August 1923)]
#115-116: [apparently derived from Shiel’s Pale Ape, cf. HPL to FBL, October 7, 1923 (SL1.255) & Eyrie._ (Jan. 1924)]
#117: A secret living thing kept and fed in an old house. [derived from Stoker’s The Lair of the White Worm? (cf. SL1.255); “The House of the Worm” / “The Shunned House”?]
1924 [before #118]
#123: Dried up man living for centuries in cataleptic state in ancient tomb. [“The Survivor”?]
#128: Individual, by some strange process, retraces the path of evolution and becomes amphibious. [. . .] [“The Survivor”?]
#130: N.E. region call’d ‘Witches’ Hollow’—along course of a river. Rumours of witches’ Sabbaths and Indian powwows on a broad mound rising out of the level where some old hemlocks and beeches formed a dark grove or daemon-temple. Legends hard to account for. [derived from Oliver Wendell Holmes’s The Guardian AngelI?—read by HPL on March 28, 1926 (cf. NY.287)]
#133: Man has a miniature shapeless Siamese twin—exhib. in circus—twin surgically detached—disappears—does hideous things with malign life of his own. [used by Whitehead in “Cassius” (cf. e.g. SL5.33-34)]
#134: Witches’ Hollow novel? Man hired to teah in private school misses road on first trip—encounters dark hollow with unnaturally swollen trees and small cottage (light in window?). Reaches school and hears that boys are forbidden to visit hollow. One boy is strange—teacher sees him visit hollow—odd doings—mysterious disappearance or hideous fate.
#139: [the Delrio quotation used in “The Horror in the Red Hook” (Aug. 1-2, 1925), hence July 1925 at the latest]
[gap in use? date 1926 after #140]
#(140)-141: [extract from H. Rider Haggard and Andrew Lang’s The World’s Desire; cf. HPL to AWD, October 1, 1927 (ES.106)]
#143: Strange well in Arkham country—water gives out [. . .] [Fungi: “The Well”]
#144: Hideous book glimpsed in ancient shop—never seen again. [“The Book”?]
#151: [incorporated into “The Whisperer in Darkness” (started Feb. 24, 1930)]
#153: Black cat on hill near dark gulf of ancient inn yard. Mew hoarsely—invites artist to knighted mysteries beyond. Finally dies at advanced age. Haunts dreams of artist—lures him to follow—strange outcome (never wakes up? or makes discovery of an elder world outside 3-dimensioned space?) [“Old Man” cat in Providence, c. September 1928? (cf. SL5.81-83); shades of hyperspace in “The Dreams in the With House”(Jan./Feb. 1932)?]
#158: [plot derived from H. B. Drake’s The Shadowy Thing (1928) read c. early 1929? (cf. ES.173); later developed as “The Thing on the Doorstep” (Aug. 21-24, 1933)]
#169: What hatches from primordial egg [perhaps incorporated into At the Mountains of Madness]
#171: Hideous old book discovered—directions for shocking evocation. [“The Book”?]
[1930]
#178: A very ancient tomb in the deep woods near where a 17th century Virginia manor-house used to be. The undecayed, bloated thing found within. [plot for a "Randolp Carterish" story, cf. undated postcard to CAS quoted in the notes to Smith, The Door to Saturn]
#181: [plot suggested by CAS in a letter of c. October 24, 1930 (SL.128)]
#182: [the kernel of “The Shadow Out of Time”; cf. HPL to CAS, November 11, 1930 (SL3.217)]
#184: [perhaps incorporated into At the Mountains of Madness; thus no later than January 1931]
#185: Scene of an urban horror—Sous le Cap or Champlain Sts.—Quebec—rugged cliff-face—moss, mildew, dampness—houses half-burrowing into cliff. [Quebec first visited in August 1930]
#187: Dream of awaking in vast hall of strange architecture [. . .] mind has become transferred to body on another planet [evidently incorporated into “The Shadow Out of Time”; cf. HPL to CAS, November 29, 1930 (SL3.238); also HPL to CAS, October 22,1933: ‘about a year ago’ (Dream Book #14])]
#192: [derived from Alexander David-Neel’s Magic and Mystery in Tibet (1932)]
#195: Pane of peculiar-looking glass from a ruined monastery reputed to have harboured devil-worship set up in modern house at edge of wild country. Landscape looks vaguely and unplaceably wrong through it. It has some unknown time-distorting quality, and comes from a primal, lost civilisation. Finally, hideous things in other world seen through it. [“The Rose Window”?]
#198: Distant tower visible from hillside window. Bats cluster thickly around it at night. Observer fascinated. One night wakes to find self on unknown black circular staircase. In tower? Hideous goal. [evidently incorporated into “The Haunter of the Dark”]
[AMS with entries up to #201 presented to R. H. Barlow on May 7, 1934; #202ff. in the typescript folder]
#209: [derived from an article in the NY Times of March 3, 1935]
#221: [the last entry, derived from a dream; cf. HPL to R. H. Barlow, (May 11?) 1935 (FF.261)]

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Juha-Matti