There are three books that I found quite useful.
- McKeachie's Teaching Tips - this is small but dense, packed with good ideas (good to re-read from time to time)
- The Joy of Teaching by Peter Filene - good for new teachers
- Teaching Unprepared Students, by Kathleen F. Gabriel - good if you have students who are not really strong academically and you want to help them
Aside from books, you could consider getting a PGCHE (post graduate certificate in higher education) which is normally a 600 hour / one year course specifically on teaching to adults. You can get one via distance learning, if you're not in the UK.