Even though Amazon.com has used book links, I prefer to go directly to the source:
They are running a referral
campaign that gives you a 15% discount on your first order.
Shipping is free if you spend $10 or more. I've gotten nothing but good quality used books from them and,
did I mention that shiping was free? So a one penny book at Amazon costs $4.00 to ship so a $3.59 book
from these guys is the buy.
Don't be fooled by 1 penny books, books are heavy and costly to ship. Thrift Books has great prices,
an incredible selection and when you add up the total cost on other sites, they have
come out ahead enough times that I don't bother shopping anywhere else.
After you buy $50 in books, they send you a discount code for $5 off your next order (roughly 10%).
Thrift Books also offers an occasional discount, but it amounts to a few pennies.
Recently they've been running a promotion: 2 for $7, 3 for $10, and 4 for $12 which puts them
in line with AbeBooks below.