This post has been included in my new blog at http://welovearabic.wordpress.com
By this I mean, students of Arabic who have learnt a little, perhaps in a short course, and want to continue independently.
Before tackling these books, you'll probably want to have learnt to read and write, got to grips with a little of the basic grammar: feminine and masculine nouns, the fact that adjectives and nouns agree in gender and in being definite/indefinite, using possession (somebody's thing - ie. the idaafa construct), past and present tense verbs, that sort of thing. Then you'll be ready to increase your vocab and the range of situations you can use Arabic in.
Mastering Arabic
Even if you've met the basics of Arabic, and can read the script, I'd still recommend Mastering Arabic as a revision aid, and to progress beyond the basics confidently. If you find Mastering Arabic 1 too easy, it's probably time for ....
Mastering Arabic 2
This is a truly amazing and brilliant book, aimed at GCSE level and beyond to AS. You will certainly want to be comfortable with the basics, because the texts in this are quite long and introduce a lot of vocabulary to take in for each exercise. If you've picked up conversational Arabic while living abroad, if you speak Arabic as a mother tongue, or have studied for more than a year or two, then you will probably be ready for this book.
Build Your Arabic Vocabulary
This wondeful book systematically teaches you 1000 words in 16 chapters, perfectly arranged for GCSE topics. It comes with cut-out-able flashcards, and has lots of fun exercises and prompts for speaking exercises, but the downside is that there's no recording with it.