This is a collection of John Bellairs' works that were not explicitly written for children. It includes The Face in the Frost; the first 150 or so pages of the unfinished sequel manuscript, The Dolphin Cross; St Fidgeta & Other Parodies; and The Pedant & The Shuffly.
The Face in the Frost is one of the best books I have ever read, and I am so glad it's back in print now. It's at turns hilarious & very scary, full of silly obscure magicians. A wizard called Prospero (not the one you are thinking of either) lives in a faraway land & his best friend is Roger Bacon (who is exactly the one you are thinking of, unless you are thinking of Francis). Someone - or something - starts trying to kill Prospero, as well as scaring the daylights out of the entire kingdom, & Prospero and Roger must try to discover its source & stop it. Lin Carter blurbed on a previous edition that it was probably the best fantasy book since The Lord of the Rings, & I agree. (It's also completely different from The Lord of the Rings.)
The Dolphin Cross finds Prospero in a bit of trouble again, with plenty of strange happenings & more creepy animate leaves. If you've read The Face in the Frost, you've probably been wishing for a sequel for the last 20 years or however long, so this really need no introduction, except to remind you that there is no ending, it's only 150 pages & then it stops. Only Bellairs' kids books sold well enough I guess.
St Fidgeta and Other Parodies is a series of parodies of pre-Vatican II American Catholicism. I'm not Catholic & I'm pretty sure I was born after Vatican II, but I think they're funny. From the adman brainstorming how to run the Pope's visit to NY to the wonderful parody of church architecture, it's all interesting and ranges from mildly funny to LOL.
The Pedant and the Shuffly is Bellairs' other classic besides Face in the Frost. It is an illustrated novella about Sir Bertram Crabtree-Gore, Esq, who encounters an evil pedant named Snodrog who uses faulty logic to make people believe they don't exist, at which point they turn into small linen napkins which float about and do his will. Obviously Sir Bertram can't let this situation continue, and goes to enlist the help of a Shuffly to defeat the pedant. The best parts are the parodies about obscure academic & religious things, as always in Bellairs.