Udumalai.com is a good place to buy Tamil books online.
I was impressed with the easy to use web interface, Payment options and the Wide range of collection they have put up online. Very impressive and useful to anyone who would like to order Tamil books online.
On the downside, The information available in the site is not up to date. I ordered a book which was mentioned as "Stock available" in the site. But was not shipped to the given address even after 1 week. My email did not had a response on time. Finally they informed me that the book is out of stock, But still the book is listed in the site as "Stock available".
I had to order a different book as I had already made the payment. I believe, that the service should concentrate more on customer experience rather than on making the price of the books competitive.
I would recommend this service to my friends as there are not many such services currently available.