You can use both soft or hardcover books. You can even lay them in the slot sideways (binder down) if your book isn't wide enough to fit that slot standing up like normal. Hardcover works good for large book slots generally since they prop up nicely. Softcover works best for regular/medium size slots.
If you use hardcover you want to make sure they’re not too large for whichever slot because they have no bend, and if they're too large you may get jams or coils popping loose from too much tension etc.
With hard cover books you'll want the book to be a half inch to an inch less wide then that slots max width for best results. (see pic below for dimensions)
With soft-cover you can pretty much go to max width since they have a little bend to them.
But either works fine, you just dont want to mix hard and soft cover books back to back in the same slot. You can use both types though no problem.
