Some new believers are just not readers at all, so the most important thing is to encourage them to read the Scriptures. For that reason, you might want to find a book for them that would give them simple help.
The most obvious answer to that question in this culture is that Tabletalk may be the best “book.” A subscription to Tabletalk means that this new Christian will be led on day by day in the study of Scripture and also be able to read short articles on different aspects of the Christian faith and different doctrines. So sometimes when we ask the question, “What book?” maybe a book will not be the answer. Something like Tabletalk might be.
If they’re intelligent students, one of the books I would really recommend to them is R.C.’s book Everyone’s a Theologian, because at a very accessible level it gives a real overview of the whole of the Christian gospel. I think it’s a superb book for that purpose. But again, I think you just need to know the individual.