Here's a lecture by Rev. Paul Scalia on the Errors of Modernism, suggested by reader Michael Dowd. It is well worth the 48 minute run time, especially if you are not that familiar with this crucial battle for the heart of the Church. The Bear would probably find more Modernism at work in the past few decades than Fr. Scalia would credit, but Bears have an incredible sense of smell. They can literally sniff out heresy. (It smells like the remains of a cold, wet campfire on a hot, humid day.) And Modernism is nothing but, no matter which of your favorite recent popes have turned a blind eye to it.
Another good place to start (and revisit from time to time) is Pope Pius X's 1907 encyclical Pascendi Dominici Gregis, which is all about Modernism. It isn't short, but they didn't have to bind it and sell it as a book, either.