Athenaze: An Introduction to Ancient Greek Book II

I used both book I & II (along with a resource called The Perseus Project [...] which is a FREE collection of original texts & translations) in college and of all the languages I've studied I found Ancient Greek to be one of the easiest to learn, despite its difficulty, because of this wonderful textbook. The layout is straightforward, and simple. It gives many exorcises for passive and active translation. It explains the grammatical concepts well and provides a great dictionary in the back pages along with pages of tables for quick reference (the ones in this text I think are presented better than those in the Oxford Grammar Reference, especially for verbs). It is an ideal book for either a student learning in a classroom setting or someone learning on their own.
If all you want is to gain a basic understanding of Ancient Greek and the ability to do passive translation, the is almost all you'll need (except for a general dictionary once you get more into it.)
If you want to study the language extensively this is a good starting point. I have been studying Greek for around 4 years now, having progressed enough to do independent translations of actual texts, and I still use these books to review with.In addition to this I would recommend getting/using:
The Perseus Project (This is a collection of FREE texts, maintained by Tufts University, with English translations, as well as books on grammar, etc. Each Greek word is hyperlinked and clicking on one it takes you to the dictionary entry that lists what exact case/declension/other grammatical identifiers. There is also a feature that allows you to view the original text and the English translation side by side. And it also has Latin texts) [...]
Oxford Grammar of Classical GreekPocket Oxford Classical Greek Dictionary
Greek Key Words: The Basic 2, 000 Word Vocabulary Arranged by Frequency in a Hundred Units, with Comprehensive Greek and English Indexes
Lexical Aids for Students of New Testament Greek (this has some very good charts in the Appendices, especially one of prepositions)
An Intermediate Greek-English Lexicon: Founded upon the Seventh Edition of Liddell and Scott's Greek-English Lexicon
Athenaze: An Introduction to Ancient Greek (Workbook II)