Everything that affects bows affect crossbows as well (damage multiplier etc), even `Deadly Aim`. Those perks that aren't immediately obvious work like this:

 - `Hunter's Discipline` - more bolts (from mobs who drop bolts in the first place)
 - `Ranger` - move faster while aiming and reloading
 - `Quick Shot` - faster reload

Update:
The mechanic of the weapons can be confusing. 

 - Bow: Ready-draw-aim-shoot-ready.
 - Crossbow: Ready-aim-shoot-reload-ready.

Quick draw yields faster draw for bow and faster reload for crossbow. Ranger lets you move faster while aiming with your bow or crossbow. The thing with a crossbow is that you aim an already loaded weapon and with the bow the draw action is part of the aiming. In the end, it's no real difference.