Amphigorey by Edward Gorey. It's going to be a bugger to list, because while it is one volume, it is also a collection of 15 books, previously published separately as standalone works. Do I list it as one book, or as 15?
When I have come across such volumes in the past, I have listed them as separate books, e.g. The Once and Future King as 4 books and The Gormenghast Trilogy as three, but the Gorey books are short works and the whole volume is perhaps 200 pages all in all (it's unpaged, so this is an estimate based on thickness).