When is it safe to sell books and contracts?

I'm sure most people have answered this for themselves (because this inventory UI is so limited), but now that I have dozens of books and contracts in my inventory, am I safe to vendor trash all of them once I read them? Is there any risk to shedding one that I'll need later?

I did trash many books and notes on my first playthrough, and I didn't notice any case where I needed something later. As you said, you should read them all beforehand as they might trigger quest advances or give you bestiary entries.

One item I'd be careful is the pass to get across the Pontar, it looks similar to other notes and you might want to keep it.

You could also restrict selling books and notes to one particular merchant, that way it is easier to buy something back if you actually need to.