Book: Tower Allies.
Master Rahool's insatiable curiosity drove him to the Tower, where, as resident crypto-archaeologist, he can work directly with Guardians returning from the frontier. He decrypts matter engrams as a free service, and when he builds trust with a particular Guardian, he is happy to offer rare engrams for sale - although the scarcity of these artifacts forces him to ask for Glimmer in compensation.
Rahool's true love is history. He treats each new find as a chance to understand the glory of the Golden Age or the terrible truth of the Collapse. Listen carefully to his murmurings: he may be the first to understand.