Like most other MMORPG guides, there are both official guides and unofficial guides. Additionally, there are also purchasable guides versus free guides. Most of the free guides are shared online in blogs and websites that are dedicated to the game. So far some of the promising ones are those like http://www.teraguide.net and http://www.teraonlineguide.net. Paid ones usually comes in digital formats such as PDFs. When contemplating guides, it all depends on the level of depth of the content and strategies you are looking for. If you need basic info, stick to the free ones, but if you're looking to stand out in the game, then you definitely need the paid strategy guides.