The most wonderful thing about our bookcases is the fixed shelves are constructed with sliding tapered dovetails, which is the best possible way to protect the sides from gradually bowing out when you stuff it full of books like I do.
Small (27w x 25.5h x 7.5d), 2 fixed shelves, perfect for paperbacks, shown
Medium (31w x 52h x 11d), 1 fixed shelf, 2 adjustable
Large (35w x 75h x 12.5d), 1 fixed shelf, 3 adjustable