Many people do not understand what Fair Market Value is. They assume that if Goodwill sells their donation for a buck, then that is FMV. Not quite true. FMV is the price paid in a transaction between a willing buyer/willing seller, both with relevant knowledge of the facts. It also assumes no extraneous motives such as family or charity.
A more accurate valuation is that which for-profit sellers are selling similar items for (make sure they're actually selling at that price, not just sitting on the shelves.) This can be hard to find out for many things, much easier for books.
For hardcover books, I go to amazon.com. No better valuation than massive online retailers. I search for the title and author of the book and then find a used book of similar condition to mine. I ignore the $1 or $.01 sellers - they make money by overcharging for shipping and handling. I also ignore any price over cover price unless the book is clearly collectible (and I have held it for at least three years before donating it.)
I then add that book to my cart and repeat until they're all in. I print the cart and then delete the contents.
You would be amazed what a box of good hardcovers is worth.
Don't bother with this for obviously cheap, crappy paperbacks.
A bit harder but more accurate is www.bookfinder.com - use this for more valuable hardcovers. You'll need to print values individually, or save pages to your computer.