I used the Smart Choice series for a number of years at my old university for a supplementary program and really enjoyed teaching it. Smart Choice makes an excellent pick for an esl textbook for adults. It has a good bit of grammar, but in a very simple, easy to understand way and not too much is packed into each lesson. It made teaching easy and very minimal prep was required because the book was good enough to just teach straight from it. The students usually seemed to enjoy the topics and it was easy enough to build natural conversations from the content in the book.
Although it is a 4-skills book, there are plenty of speaking activities and in particular, the “free-talking” section at the back of the book is excellent. If you’re looking for a speaking textbook for adults, check it out.