Anyway, Via Boinboing, I have learned a little about Lovecraft's commonplace book.
This book consists of ideas, images, & quotations hastily jotted down for possible future use in weird fiction. Very few are actually developed plots—for the most part they are merely suggestions or random impressions designed to set the memory or imagination working. Their sources are various—dreams, things read, casual incidents, idle conceptions, & so on. —H. P. Lovecraft
Presented to R. H. Barlow, Esq., on May 7, 1934—in exchange for an admirably neat typed copy from his skilled hand.
1 Demophon shivered when the sun shone upon him. (Lover of darkness = ignorance.)
2 Inhabitants of Zinge, over whom the star Canopus rises every night, are always gay and without sorrow. [x]
3 The shores of Attica respond in song to the waves of the Aegean. [x]
Note the "May 7, 1934". The list of ideas, which totals over 200, also includes dates and 1935 is listed. Now, that is spooky.
Not really related: Rudy Rucker has published his notes and ideas for Mathematicians in Love (PDF). These notes were collected, I presume, after he published the book and clearly collected after he had the main ideas for this book. It is not a commonplace book as I understand (poorly) the term as the notes comprise the time after 'the light turned on'. Still, it is interesting to see how he put the book together.
I have not read the book, but some people (I forget who) at Boingboing love him. I suspect the PDF has spoilers so it might be wise to read the actual book first. Mathematicians in Love at Amazon (published in 2008 with no Kindle edition available for me -I'm listed as Canadian - but perhaps for Americans?)