Yes, provided that the package is at least 3/4 of an inch thick ("at its thickest point"), according to USPS Delivery Confirmation Regulations, and is being sent to a USPS-validated address.
- "rigid" (most books are)
- be greater than 6" x 3" x 1/4 " in dimension
- weigh more than 6 ounces
If you are concerned that your package does not meet the dimensional criteria above, you can always "bulk up" its dimensions with lightweight material that will not add significant weight (for example, Styrofoam, crumpled plastic bags or paper).