I'll take any help I can get! I read a short story horror book when I was young but I don't remember the title or author. I believe it was a 90's book with a purple or dark cover with a creepy black tree. Two of the story names I can remember are "The Thing in the Cellar" and "The Whistle". I've done countless searches with no luck.
There are a few possibilities:
There is a book titled Tales From Underwood published in 1952 which contains a story titled "The Thing in the Cellar" by David H. Keller, originally published in 1932. While this anthology does not contain a story titled "The Whistle" it does feature stories with similar titled (e.g. "The Doorbell"). According to the Internet Speculative Fiction Database, this story has also appeared in a variety of other horror anthologies, and while none of those listed features a story titled "The Whistle", The Century's Best Horror Fiction 1901–1950 features a story titled "The Whistling Room." Here is the cover to this last volume:
There is also a story titled "The Thing in the Cellar" by Blake Smith, published in Weird Tales Fall, 2012. There is no tale titled "The Whistle" or similar in this collection.
There are in addition a number of short stories titled "The Whistle" by different authors including, Ralph Milne Farley (Weird Tales, November 1932), Lenora K. Rogers (Dark Matter #3, 1998), M. R. James (Room 13, and Other Ghost Stories, 2005), and Hugh Walpole (All Souls' Night, 1933).
There are also stories titled "The Whistler" by August Derleth (Weird Tales, 1930; and That is Not Dead: The Black Magic & Occult Stories by August Derleth, 2005), and Dana Pearson (Northern Haunts, 2009). Northern Haunts also features a story titled "Down the Cellar" by Joseph Grant. Here is its cover art:
I set out searching for this last night too and your description matched the book I was searching for. Finally came across it today. I think you’re looking for World’s Best “True” Ghost Stories by CB Colby.