I have done several author portraits and they usually end up no more than 4" on the back flap. Sometimes they will print them on the back cover but that is becoming rare. Most likely it's the back flap.
The most recent one I've done was shot on film and the neg was scanned. That's what I would suggest as it allows you to do the kind of retouching that most people now expect: fix eye bags, remove any big wrinkles, spots, etc. You could, of course, scan a glossy 8x10 print and do the same thing as well.
If you don't want to do this kind of retouching work, feel free to shoot me a PM and I can either help myself or suggest someone for you.