If an enchanting table is surrounded with 15 bookshelves torches placed next to the table should reduce the enchantment level by 2 per bookshelf that is shielded.  However, if a single torch is placed on one of the flat sides of the table the reduction is only 1. Thus the achievable enchantment levels are 30, 29, 26, 24 etc.

This is not a big problem in practice but is probably not working as expected.