Sometimes you need to read in more than one book at the (more or less) same time - because, for instance, you want to compare stuff. I know that I've had space issues when trying to check things in several books at once.
Well, turns out that there's a solution for this, and it can be found in a library in Mexico. It's a book wheel - something looking a bit like a rolodex (for those of you old enough to know these), with several shelves for one book each, and you can turn the wheel to look at the books in turn.
Go see the thing in the original article about it. Unfortunately, I don't parallel read more than two books often enough to justify trying to build one of those...