I would not want to carry a backpack for that distance, but I also agree it's sometimes a shame to put a rack on your light fast bike. Having to carry books makes a rack more attractive IMO.

For a light commuting load - just clothes - hanging a bag in the front triangle turned out to be the most convenient for me. I'm also experimenting with an aerodynamic handlebar trunk, but it wouldn't accommodate books. I really think for 106 km and some rougher road surface the most reasonable option is installing a rear rack. Select a bag for whatever's best for carrying off the bike, and just bungie cord it on the rack. Having also a cargo net is convenient for light loads or additional stuff.