SOLVED. YA horror book from the 80s [s]

[Greyflood]

Hello. I'm trying to track down a book my friend is in search of from his childhood. He read it in the late 1980s, though it might be older as he found it in a public library. He believes it is a young adult novel, though it might be normal adult fiction too (he was an advanced reader for his age). It was a horror story with many elements reminiscent of H.P. Lovecraft. Here are some of the details he remembers:

--A boy in the story becomes ill with a mysterious disease. His friend and a professor of some kind have to try and save him by investigating a strange church.

--The sick boy found a paper mask of his own face that may have been what made him ill.

--The friend and the professor discover a cult hidden beneath the old church in underground tunnels.

This book might have been part of a series, or at least one of several such horror books by the same author. The cover featured a stained-glass window with strange imagery on it.

We would be very grateful if anyone can help us retrieve this book from the memory hole!

[Angela]

The boy and a professor makes me think of the Johnny Dixon/Professor Childermass books by John Bellairs. I'll keep searching and see if I can figure out which one (if any) this is.

edit: Perhaps this one: The Revenge of the Wizard's Ghost

Collapsed in a weird trance, Johnny Dixon is dying. Professor Childermass and Fergie soon learn the horrifying reason: Johnny is possessed by the spirit of the evil Warren Windrow. In a desperate effort to find a clue that will save Johnny's life, the professor and Fergie race to the gloomy Windrow estate. But demonic forces are waiting for them, and suddenly they're fighting for their own lives.

[Greyflood]

Jackpot! That's the one! Our many thanks for the super-speedy and laser-beam accurate response!

[Angela]

Great, glad to be of help!

[Kris]

Big recommendations for the series! There are a couple not so scary books but Eyes of the Killer Robots made me at age 24, go read in the bedroom where my boyfriend was sleeping.

John Bellairs is a favorite of mine even at 46.