JW's do not use any literature not published by the Watchtower Society. Commentaries are included in this ban, although the WTS quotes from commentaries at times. Their explanation is that the WTS has already done the studying of these commentaries and other books for them, so all they have to read is WT material and that's enough. They can still use the "Aid to Bible Understanding" book, though I believe they use "Reasoning from the Scriptures" more because it's newer and doesn't have all the "old light" that's still in the Aid book.
Jay, the book your host family was using was probably the Yearbook. It has all the info from the previous year (number of witnesses worldwide, growth, stuff like that. They are very focused on numbers), plus a devotion for each day. Again, they aren't allowed to read or even own a book of devotions, even if all it quotes is Bible verses, that isn't published by the WTS.
Hope that answers your question Batty.
"He saved us, not because of deeds done by us in righteousness, but in virtue of His own mercy, by the washing of regeneration and renewal in the Holy Spirit" - Titus 3:5