1. Look at the displays. Libraries often are promoting a theme and have some displays out.
2. Look at what is being returned. My local library is small so the to be re-shelved cart is accessible. Since this is a random mix of books you end up looking that things you never would have considered normally.
3. Look at the new arrivals. This again is often a random mix across many genres.

At this point I then start sidestepping. Either I spend some time going deep into an author's back catalog or I see what is shelved nearby.

Also, a lot of imprints develop a certain style. If you notice that a handful of books you liked recently have the same publisher start looking for that publishers mark on books.