Alibris, which sells new and used general books and textbooks, has the following coupons good through the end of March, 2010:

$2 off orders of $20 or more via coupon code "MEMPHIS"
$3 off orders of $30 or more via coupon code "DOIG"
$5 off orders of $50 or more via coupon code "LAMB"
$7 off orders of $70 or more via coupon code "MUN"
$9 off orders of $90 or more via coupon code "GLASS"
$15 off orders of $150 or more via coupon code "KALLOS"