I use them often to mail books. They are the right size for paperbacks. I wish you sold #1 envelopes as I use them a lot for larger books that are too small for #2 envelopes which have to be re-sized every time I mail them resulting in my often using products found elsewhere. Also not sure about their being water resistant and how secure the closure so I usually tape the corners.
The square shape allows you to address them in any direction without the post office charging extra. Did you know that if you place your address label horizontally with the short side of a large rectangular envelope it is considered a package and the rate is higher.