The Horror in the Museum by H.P. Lovecraft and Others

The Horror in the Museum  by H.P. Lovecraft and Others (1970/1989/This edition 2007): edited by August Derleth, Stephen Jones, and S.T. Joshi:

Primary Revisions: Which is to say, stories that are almost entirely rewritten by HPL from stories or notes from other writers.

• The Green Meadow (1918) by Winifred V. Jackson and H. P. Lovecraft [as by Lewis Theobald, Jr. and Elizabeth Neville Berkeley]: Really a Dunsanian prose-poem more than anything else.
• The Crawling Chaos (1921) by Winifred V. Jackson and H. P. Lovecraft [as by Lewis Theobald, Jr. and Elizabeth Neville Berkeley]: Again, really a Dunsanian prose-poem more than anything else.
• The Last Test (1928) by H. P. Lovecraft and Adolphe de Castro [as by Adolphe de Castro] : Enjoyable, overlong novella about a scientist's descent into madness, his descent speeded by the advice of a monstrous survival from eons past. Set primarily in late-19th-century San Francisco.
• The Electric Executioner (1930) by H. P. Lovecraft and Adolphe de Castro [as by Adolphe de Castro]: So far as I know, the only story written or rewritten by HPL to be set primarily on a train. 
• The Curse of Yig (1970) by H. P. Lovecraft and Zealia Bishop [as by Zealia Bishop]: Should be moved to the HPL canon. One of Lovecraft's generally top-notch "collaborations" with Zealia Bishop that moved the Cthulhu Mythos into the Midwestern environs of Oklahoma.
• The Mound (1940) by H. P. Lovecraft and Zealia Bishop [as by Zealia Bishop]: Another top-notch, almost-canonical Lovecraft-Bishop Joint. A secret land of Great Old One worshippers hides about a mile below the surface of Oklahoma. Well, that explains a lot! 
• Medusa's Coil (1939) by Zealia Bishop and H. P. Lovecraft [as by Zealia Bishop]: Oh, Lord. Brace yourself for the most racist ending in Lovecraft's stories and revisions, so anomalously ascendant over much more dire information that the ending almost seems like a parody.
• The Man of Stone (1932) by H. P. Lovecraft and Hazel Heald [as by Hazel Heald]: Minor horror stuff.
• The Horror in the Museum (1933) by H. P. Lovecraft and Hazel Heald [as by Hazel Heald]: Solid Mythos material set at... a wax museum?
• Winged Death (1934) by H. P. Lovecraft and Hazel Heald [as by Hazel Heald]: Somewhat goofy Africa-set horror story.
• Out of the Aeons (1933) by H. P. Lovecraft and Hazel Heald [as by Hazel Heald]: Another piece that could be considered canonical Mythos horror.
• The Horror in the Burying-Ground (1937) by H. P. Lovecraft and Hazel Heald [as by Hazel Heald]: Very minor horror material.
• The Diary of Alonzo Typer (1938) by H. P. Lovecraft and William Lumley [as by William Lumley]: Promising beginning, somewhat muted ending to a haunted house story with shades of the Great Ones looming behind it. Bears more than a passing resemblance to Stephen King's early-career homage to HPL, "Jerusalem's Lot."

Secondary Revisions: Which can range from a lot of HPL to almost none at all.

• The Horror at Martin's Beach (1923) by H. P. Lovecraft and Sonia Greene  (variant of The Invisible Monster) [as by Sonia H. Greene]: Minor horror stuff with hypnotic sea monsters.
• Ashes (1924) by C. M. Eddy, Jr. and H. P. Lovecraft [as by C. M. Eddy, Jr.] : Minor science-fictional horror that seems inspired by Robert W. Chambers.
• The Ghost-Eater (1924) by H. P. Lovecraft and C. M. Eddy, Jr. [as by C. M. Eddy, Jr.] : Really minor lost-in-the-woods horror.
• The Loved Dead (1924) by C. M. Eddy, Jr. and H. P. Lovecraft [as by C. M. Eddy, Jr.] : Oh, brother. Controversial (for its time) story about a guy who, though his actions are never completely described, seems to be a necrophiliac.
• Deaf, Dumb and Blind (1925) by C. M. Eddy, Jr. and H. P. Lovecraft [as by C. M. Eddy, Jr.] : Minor piece of cosmic horror.
• Two Black Bottles (1927) by H. P. Lovecraft and Wilfred Blanch Talman [as by Wilfred Blanch Talman] : Droll tale of zombies and churchyards.
• The Trap (1932) by Henry S. Whitehead and H. P. Lovecraft [as by Henry S. Whitehead] : Minor piece of horror revolving around optics.
• The Tree on the Hill (1934) by Duane W. Rimel : Under-developed piece of cosmic horror.
• The Disinterment (1935) by H. P. Lovecraft and Duane W. Rimel [as by Duane W. Rimel] : Minor horror that seems mainly inspired by Poe.
• "Till A' the Seas" (1935) by H. P. Lovecraft and R. H. Barlow  (variant of "Till All the Seas") [as by R. H. Barlow] : Downbeat fragment of end-of-the-world melancholy.
• The Night Ocean (1936) by H. P. Lovecraft and R. H. Barlow [as by R. H. Barlow]: Probably the "weirdest" tale here in the modern sense, as atmosphere and suggestion take front place over specific, horrific occurrences. Extremely strong piece. 

Overall: Essential to the Lovecraft fan and/or scholar, and with enough rewarding tales of horror and the macabre to satisfy the casual reader as well. Highly recommended.