I've had to deal with this a lot and here are some things I did:

- Upgraded all my bookshelves (from 5-shelf bookshelves to slightly wider 7-shelf bookshelves)

- Bought more bookshelves for wherever there was wall space

- Bought a bunch of airtight containers and put books I'm less likely to read away in the basement (they're inventoried for easy search)

We also bought doors for all our bookshelves at one point, but that's more about book care than increasing book capacity.

Right now I actually have bookshelf space for more books, but I'll probably need to go through the containers in the basement and actually get rid of some books eventually. :(