SE-ED bookstores are found in many departmental stores and hypermarkets (like certain large Tesco or BigC outlets). In the past when I visited Thailand, I would buy the calendars and work diaries from bookstores such as SE-ED or Naiin.
Above is SE-ED store located at Central Departmental store.
Usually the bestsellers in Thailand are sold in this bookstore. These consisted of the Thai bestsellers as well as popular English bestsellers that are translated in Thai language. There are also some books in Thai language being sold here as well.
I’ve always like to go straight to the section where they sell Thai dictionaries and phrase books. Over the years I must admit I’ve amassed quite a number of such books. But nowadays I am learning through speaking with my Thai friends.
For example the above are popular books written by Thai authors. YouTube and Facebook are two of the very popularly frequently sites in Thailand. I was told that blogs are not so popular with Thai people- they prefer to go to Facebook to search for the items that they need rather than visiting blogs. That is why particularly all companies in Thailand would focus on starting a Facebook page than a blog. So naturally there would be books in Thai language teaching on how to optimise for Facebook and YouTube.
Students tend to visit SE-ED bookstores so SE-ED would carry various types of textbooks and stationery items for school. There are toys and gift sets being sold as well.
You can checkout the SE-ED store listing from here. Well, the website is not fully English optimized but the GPS coordinates are provided in their store/ outlets listing.