I actually tried reading two books at once on two different occasions. The first time, both books had too many characters and I was starting to mix up which characters were in which books. The second time, the settings were so similar (castle/mansion in England/Ireland) that again I was mixing up details.
If I immediately start a new one depends on how the previous one hit me. Sometimes a book can affect me in a way that I need to sit with it a bit, let it settle, because I find myself thinking of it still when trying to start something new. But if it's just a breezy read that doesn't linger on my mind then I can easily start something else. "The Buffalo Hunter Hunter" had such an unsettling ending that I couldn't start anything new for like a week or more. My brain wouldn't move on from thinking about it. I had a similar experience with "A Head Full Of Ghosts". That ending still sticks with me. I heard it's being made into a movie and I just know Hollywood is going to screw it up so bad.
I've definitely read books like that, ones where the ending hits so hard, I have to ponder it for days. And I've definitely read books that I learned would be turned into a movie and I was thinking "please don't blow it!".
