You have to buy a map of Bangkok if you be there. I buy this map from Siam centre and have inside all Bangkok, all the streets (Thanon) and the small streets (Soi).
First Photo: Bangkok city atlas
What to buy: Map of Bangkok
If you love reading then head to the Kinokuniya Bookstore at the Siam Paragon. It is like being a kid in a lolly store. There are different language sections - there is a large section for Thai, Japanese and English books and smaller sections for other languages. They have a huge selection of magazines as well and there is also a coffee shop attached
I'm a huge fan on Books Kinokuniya, the Japanese bookstore chain usually found nearby Isetan department stores. My experiences are mainly from Singapore -- and unfortunately their Bangkok branch was a disappointment.
The English section was quite limited, I was expecting bigger selection in art, architecture, design and photography, to name a few, but the shelf space devoted for these were really small -- especially compared to the store in Singapore.
The whole store felt worn and did not invite to spend time with the books. This all said, the store is still worthwhile visit for buying novels, maps and so forth.
A refreshing change from the other formulaic book outlets in town - they stock a lot of in-depth material on SE Asia and Asia generally as well as a great section on Buddhism. They also have a small antiquarian selection.
What to buy: They stock all sorts of great books on Asian topics - art, history, religion, politics, natural history etc .... many titles that can't be found anywhere else in town. And the shop covers topics not just for SE Asia but for all of Asia - for example they have a lot on the Himalayan region, too. (But not for popular fiction, cookbooks or newsmagazines).
What to pay: Baht 200 - 2,000.
Bookazine, as the name suggests, has a good stock of English language magazines as well as a great selection of books. You get a slightly smaller book selection than in most branches of Asia Books, but this is made up for in most branches by the addition of local and international newspapers.
There are several branches of Bookazine around Bangkok (including two on Silom Road), but the one I usually go into is pictured. This branch is on Sukhumvit, next to Soi 5 (where I usually stay), and if you want to stop and read your purchase straight away just nip next door to Starbucks, buy a coffee and read away...
Unlike the Asia Books chain, there are also Bookazine locations in some of the larger tourist resorts outside Bangkok - Pattaya, Hua Hin, Patong in Phuket and Chaweng in Samui all have branches.
If you're looking for a cheaper deal on fictions or travel guide books, check out Khao San Road. There are several secondhand book shops along the street.
What to buy: You can find a travel guide book for your next destination here for like half price. Some of them are still in good condition. Anyway, the price tells.
And if you dont want to carry the books you finish anymore, you can also get rid of them here. Sell them to one of those shops.
Want to buy an English language book in Bangkok? If you do, it's worth checking out Asia Books. As well as imports, there's a surprisingly large number of locally-published English language books - very useful if you want to delve deeper into the local culture. They also have great "coffee-table" books which make nice souvenirs, both on Thailand and neighbouring countries.
There are branches of Asia Books on Sukhumvit Road, in the Siam Discovery Center, the World Trade Centre and the Emporium shopping mall. Take a look at www.asiabook.com/store/maps.asp for branch details. The branch pictured is located at 221 Sukhumvit Road, near to the Robinson Department Store and the Asok Skytrain station. Unlike the Bookazine chain, there are currently no branches outside Bangkok, so if you see a book there you like, it may be wise to get it before you leave town.
Incidentally, although you can get books on your way home at Bangkok airport, they are much more expensive there than the shops in town - beware!