From time to time a pay a visit to SE-ED Books.
They've got a nice selection of books, divided in six levels.
These books are quite cheap, about ฿75-85.
I have been using these books for quite some time now.
A nice, and easy way to stimulate students to read.
The nice thing about these books is that they have a list of the most important words on the side of each page with their Thai translation.