There are a number of foundations books out there. The good news is that when you are a new believer the main “foundations” book you really need is the New Testament. Especially Matthew, John, and Romans. If you can get the message of those 3 books, you have a lot of the really important stuff. As our team has been working with newer believers that we cannot personally disciple we have looked for resources that we can hand to them to do some basic work on their own.
As far as “workbooks” go, the “Handbook for Christian Maturity” by Bill Bright and Campus Crusade is probably the best one available. I have looked at Derek Prince’s material, Bob Weiner, and a number of others, but Bill Bright is just very easy to understand, straightforward, etc. I like Weiner personally, but the money teaching in the book really blows it for me. Prince’s book is actually very good, but it is not simple enough for true intro material. It is a better “intro” for someone coming in from the evangelical church, not a raw recruit. A very recent alternative that I’m excited about is Larry Kreider’s “Discovering the Basic Truths of Christianity.” We may be switching to this after I have a chance to review.