Links to many good places to buy good books

 Maintained by Thai Student Association at the University of Pittsburgh
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 All-in-one pages
Addall.com The place where you can search for books, including used books, and compare prices at 41 online bookstores.

BestBookBuys.com similar place where you can search for books and compare prices at 26 online bookstores
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Classbook.com -- It sells used books as well.  $4.95 shipping per order (regardless of the number of books).

CUBook.com -- The first Thai internet bookstore (CU Book Center).  International orders accepted.  Some international-version textbooks may be as cheap as half (or less) of those published in USA.

Half.com -- A lot of cheap books, including used books, with the prices that could be less than half of their original prices.

ThaiAmazon.com -- The thailand's largestbookseller online, as they claim. :-)   contain thai books to be shipped to thai address, but maybe your folks will want to take a look or order it for their friends there.

Textbook.com -- Kind of especially geartowards students and faculty. $4.95 shipping, flat rate.

The On-Line Books Page -- Here's another gigantic resource for free.  This's a repository of FREE books online in all subjects (i.e. social science, math, etc.)
 Tips
You may read book reviews and descriptions first from Amazon.com.  Next, copy the ISBN or the title of the book you want and use it to search at an all-in-onesite, such as Addall.com or BestBookBuys.com.  Compare
prices from the search results and choose the cheapest one with your prefered shipping time. 
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If you know any good source of on-line textbooks store, please do not forget to let us know