First check out the Resources for Learning About ACT.
See the list of ACT books.
To understand more of the ACT langauge and concepts, check out the ACT Glossary of Terms.
To gain a deeper understanding about ACT and it's intellectual foundations, you may wish to browse through this section of the site: www.contextualscience.org/basic_foundations. The topics presented here are written with professionals in mind, but it is a good starting place for learning more about the roots of ACT.