Often for me it’s not about not liking the book, but not wanting to read it right now. This is particularly true with philosophy, but also true of fiction (I’m currently reading and loving Infinite Jest, but I’m not always in the mood for it).

That's not it at all. I do ditch books I'm not into. The situation I'm describing is usually where the story has some tension or I feel like the next chapter might have tension, and I don't want to push on for the time being. I need some calming space, and another story is perfect for that. Edit: I said "sick of" in my previous comment when really I should have said tired of.