I see Sibley Guides for sale elsewhere as “paperback”, “hardcover”, and “flexibound”? Which one should I buy?
They are all the same (with one exception). The Sibley Guides to Birds and Trees have only one binding, called “flexibound”. It is a flexible cover, thicker than a typical paperback and extending beyond the page edges. It is intermediate between the traditional “paperback” and “hardcover” styles, and many sellers list it as one of those two categories.
Regardless of how it is listed, all copies for sale have the same flexibound cover. The only exception is the Sibley Guide to Bird Life and Behavior, which was originally published in a hardcover binding, but is now sold as flexibound.