Crusader in dungeon, in Latin

I'm reading a very old history from the days when historians breezily assumed that their readers would be able to read Latin. The author quotes a source thus:
"..quod in hostium vinculis summo labore collegerat, litteratus modice, sed magis tamen naturali mentis vivacitate, scripturam nitebatur apprehendere."
It's about a Crusader taken prisoner by Saracens, and I get that it's about him occupying his time by studying, and something about the natural keenness of his mind, but can anyone translate it for me precisely?

I actually posted this query once before, but all the Latin scholars here must have been on their summer holidays because nobody answered. I'm hoping they've come back now!