For the most part, I already know what sort of book I'm in the mood for, and have my two TBR shelves staring down at me all the time, and it's fairly easy to just pick a book to read. It's usually down to a choice of three or four possible candidates. Once I pick up the book, though, that's the one I read.
I might occasionally read a book for one session, then when I come to pick it up again, find I'm not in the mood (once this year), or due to other commitments, have to start another instead when I'm part way through, but for the majority of times, once I start a book, I read it until I've finished (or I really can't bear to read anymore and it gets discarded - another very rare event)