No, sorry there isn't actually a book named The Mammoth Book of Karl Edward Wagner, though there should be!
Recently, I started poking around my handful of Mammoth anthologies. I have mostly horror tomes edited by Stephen Jones.
I love those things, though I tended to grab them for a single story. Going back over the tables-of-content, I am noticing authors who I am now aware of that I wasn't aware of when I bought the books - Manly Wade Wellman, David J. Schow, Hugh B. Cave, et al. (Which is why I should find more reading time.)
Some of these Mammoth anthologies have even found their way to ebook versions.
Sooo... - long story long, I thought I'd put together a list of Mammoth appearances of Karl Edward Wagner. For those who are interested in reading him but find acquiring the Wagner collections and novels difficult, maybe these can help. Some of the ebooks are pricey but there are lots of good stories alongside Wagner's tales. If not available as ebook, some can be found cheaply enough used.
The Mammoth Book of Frankenstein (Kindle)
“Undertow” (A Kane tale)
The Mammoth Book of Wolfmen (out-of-print)
(a reprint of The Mammoth Book of Werewolves (out-of-print))
“One Paris Night” (A Becker tale)
The Mammoth Book of Vampires (Kindle - yes, you can preview it, just click where the cover image would be)
“Beyond Any Measure”
The Mammoth Book of Monsters (out-of-print)
The Mammoth Book of Zombies (Kindle)
The Mammoth Book of Terror (out-of-print)
"The River of Night's Dreaming"
The Mammoth Book of New Terror (Kindle)
"The Fourth Seal"
Best New Horror 2 (Kindle) (technically, not Mammoth but edited by Jones and came up in my search)
In fact, there are also a large number of Best New Horror Stories. Wagner's name is on many of the covers, but I can't see the contents of the out-of-print ones. E.g.; number 4. (update; available for Kindle; number 3 and number 5)
Oh, of course, I just found this. I knew about it, but forget about it, too. Well, it doesn't have the Centipede collections or the ebooks or later Mammoth books, so my efforts aren't completely in vain.
Karl Edward Wagner Bibliography.
Also, according to that list, he had two stories in The Mammoth Book of Ghost Stories. (out-of-print)