Map of Thailand, Road and City
It is a good idea to buy a road map, or road atlas, if travelling round Thailand by motorcycle or car. Decent road maps of Thailand are available at bookshops selling foreign language books, particularly around the more touristy areas of Chiang Mai and Bangkok. These can save you if you get lost as many Thais speak no or little English, particularly in North Thailand, and if you do get lost, it can be a bit of a struggle to get directions. Buying a road map, or road atlas, with the names in Thai script as well as Roman would be useful. Be careful of buying poor copies of road atlases. Sometimes these are very smudged and it is hard to make out details. Check a few pages before purchasing an atlas just to make sure.
When you fly into Bangkok airport, it is worth getting maps of the city from the tourist information desks at International Arrivals - there is a map of the city showing the main tourist attractions and the embassies, and there is a map of the bus routes and public transport which is very useful for getting into the city as it much cheaper by public bus than getting a taxi - the journey takes about an hour depending on the traffic. There is also an airport shuttle.
Tourist destinations usually have tourist information offices with maps of the local area so it is worth popping in to pick up a map of the local area when you are staying in a tourist resort. Sometimes these are free, or at least inexpensive.
Most guidebooks tend to have relevant maps in them. You may need more detailed maps if travelling on your own, ie by car or motorcycle, or if visiting a small town or village which isn't large enough, or of enough interest to tourists, to have map of it in the guidebook. As above, when purchasing a guidebook in Thailand check to see if it is a copy. The maps in the copied guidebooks are very blurry. Army maps can be useful for people trekking off the road.