The quick and easy way: do everyday things like challenge logs, roulettes, etc. and use your tomes for materials to sell.
Eureka: If you have access to Pyros, bunny fates can give you some good amount of gil if you're lucky.
The longest but best investment: Crafting and gathering. People love glamours. The more you are able to gather and craft yourself, the more you can make. Some of the most expensive mats usually come from high level Treasure maps.