If you are looking online, the best you'll probably do are the Lives of Crassus and Pompey by Plutarch, a second-century Greek biographer.

If you are looking for any source, Scullard's From Gracchi to Nero is a standard textbook that gives a quick overview. Gruen's The Last Generation of the Roman Republic is a good scholarly overview of the period. He discusses their consulship early in the book.

Beyond those there are, of course, modern biographies. Pompey gets a lot of ink, but there are a few of Crassus as well. If you are looking for scholarly work, a good indication is if it is published by a university press.