Generally, books in the franchise fall under one of two categories: the Confession Rehearsal series, and the anime novelizations. There may be one or two mini novels that come packaged with CDs, but generally they aren't as frequent.
The Confession Rehearsal series is a light novel series by HoneyWorks and several authors. The novels are primarily written by Touko Fujitani until Kinyoubi no Ohayou, and Mari Kousaka starting with Heart no Shuuchou.
The Anime novelizations are also penned by Kousaka. Like the anime they're based off of, they have many similarities with the original works, but there are a few changes present.