Some of the information I've gotten comes from this great book:

Musique Fantastique, Second Edition
100 Years of Fantasy, Science Fiction & Horror Film Music
A Historical Appreciation & Overview
By Randall D. Larson

It features miniature biographies of some of the "big guns" of film composers who specialized - or are best known for - horror, mystery, adventure films of "Hollywood's Golden Age": Max Steiner, Frank Skinner, Hans Salter, etc. This is one book you CAN judge by it's cover - the Boris Karloff version of The Frankenstein Monster as an orchestra conductor. Great research and good insights - not enough on the "B-Movie" composers like Edward Kay, etc. - but worth getting for film music scholars and fans!