Keep them dry but allow air circulation and a way for any water that makes it in to get out. Plastic bags are good but they can backfire if water, humidity and condensation, collect inside and can't get out. Humidity can be an issue and cause the paper and covers to warp. In earlier times no southern home that had books was without a book press.

A field expedient version can be made by using two still pieces of plywood and using light rope to form a Spanish windless on each side to draw them together and keep the books flat. The books can be stacked together inside your press and the press can be used as a handle to move the stack. Books tightly pressed closed are resistant to moisture, insects, and fire/smoke damage. The covers of cheap hardbacks and paperbacks can bond together if moisture and pressure are combined but this can be stopped by using wax paper between books.