As you don’t know the level of the students you’ll be teaching, I don’t think it is worth your while taking only course books. If you have an absolute favourite, then do take it. However, there is a chance you won’t get to use it, if you don’t teach a group at that level.
I suggest you take teaching guides that will give you tips for teaching any level. By this I mean books about teaching pronunciation, speaking skills, error correction, managing a group of students, etc. You might also like to take books about teaching vocabulary and grammar. On 26 September 2004 I wrote about using a vocab bag. This is a practical way to teach and revise vocabulary using very little in the way of materials. There is also a game called “Word Up” which allows practice of language in a fun way; this can be ordered from the Tefl.net website. A dictionary would also be very useful.
Find out if newspapers and magazines in English are available where you will be teaching and whether you will have access to internet. If you won’t have access to any of these resources, I suggest you take some magazines or newspapers with you. At higher levels, authentic materials provide a challenge for the students. There are also activities you can do with lower levels using these materials (see my answer to Alex on 4 April 2004).
I think this is a great opportunity and I am sure you will enjoy the experience. If you would like more specific examples, don’t hesitate to write in again.