"That library contained nothing that the girl Rey does not already possess," Yoda says. It sounds cheeky, but he’s being literal: as we see later, Rey snatched the books before fleeing Ahch-To and hid them in a drawer on the Millennium Falcon (we briefly see her store them as she takes off, and get a better look near the film’s end when Finn opens the drawer to get a blanket for Rose). So yes, in addition to Luke’s lessons that Rey no doubt took to heart, she also has the literal tools to reform the Jedi Order, something Luke failed to fully accomplish. That’s another lesson Yoda imparts on Luke: "We are what they grow beyond. That is the true burden of all masters."
It’s a lovely moment between student and teacher, and shows us that even forty-some years later, Yoda still has lessons to bestow on "Young Skywalker." Considering Luke’s fate by the end of The Last Jedi -- becoming one with the Force in a sacrificial moment of heroism -- it seems likely that he’ll become her Yoda. But for now, she’s fully on her own. And because of that, we have to wonder: what is in those books and how will they help?