This is a great resource. It includes idiomatic usages and proverbial sayings, and a lot more. Once you're in the database, you will have to select French.
In addition to the books listed below, on each floor of both Raynor and Memorial, you'll find dictionary stands with a French-English dictionary. Well, that's assuming it's not already in use, nor has gone 'walk-about'!
These are the equivalents of our Webster's or Random House dictionaries.
Visual dictionaries are sometimes incredibly useful ... e.g. when you're not quite sure what to call something even in your own native tongue!
When you need to know how the French would handle a tricky grammatical situation, try one of these.
For those hard-to-conjugate verbs, here are several resources that should help ...