Harriet Hargrave's books on machine applique and machine quilting probably taught me more than any other books did (but I have tons of other books as well!).
I agree with looking through books at the library. Also look at bookstores. Sometimes a book is completely different than you think it will be; it helps to actually see it first. (Also check online. I have run across portions of Harriet Hargrave's book online.)