Oh, you may very well have a textbook, one sanctioned by the Japanese government! It'll likely be bilingual with all the directions and explanations in Japanese. Typically, a real piece of manure to use.
Your role in using it will depend entirely on your JTE, as I mentioned above. Since you are a native English speaker, you might be nothing more than a human tape recorder whose job is to read aloud the various examples, while students repeat or shadow you. Or you might actually have a serious JTE who considers you a near-equal and wants to spend time sitting down with you to plan out lessons where you create things, usually ways to get students to speak in an enjoyable way. My guess is, it's pretty much in between the two.
Upside:
You can usually leave earlier than the JTEs and have evening time on your hands for whatever you like (studying Japanese, entertainment, or more work). You will probably have tons of time on your hands during the day. Use it as you like.
Downside:
Many ALTs are bored silly with that free time during work. Depending on your dispatch company, you will probably have zero benefits and little to no pay during the months when you don't work (while the agency continues to make money from the school). You may even have a performance review from the company, but beware of little things they cut you on because they will undoubtedly mean less of a bonus or raise. You might even have your contract dropped at the last minute in order to avoid paying you a bonus. More downsides include lots of travel in some cases, to more than one school, giving you lots of traveling woes and little consistency from class to class in any one school. Since you will probably have to have an instructor's visa, being an ALT prohibits you from working anywhere else but a mainstream school...legally, that is.
