[rararational]

With my personal experience of being a student in graduate level engineering classes, one thought to consider is that while there is a load of books for undergraduate courses when you get in the tougher subjects (or more advanced subjects) there really exist little amounts of new books to teach with which leaves the burden on the teacher to create notes and problems for the students so make sure it is in a field you enjoy and do indeed know a good amount about (of course if it isn't engineering courses you are teaching it may be different).