As pointed out here, Mako filled up his bookshelf after a breakup and started devoting time to studying. Bryke also points this out in the Book 3 Commentary:
Michael Dante Dimartino: Can we talk about this little line about Mako reading Jinora’s books? It kind of came from an idea where since Mako wasn’t dating Korra and Asami anymore, he was burying himself in books and studying. Reading on the airship and stuff.