In the crowded world of Greek Grammars (Mounce cites a statistic of 10 grammars for every 9 professors) David Alan Black is a personal favourite having produced two of my favourite works on Koine Greek.
It's Still Greek To Me by David Alan Black
This is a fabulously written and very helpful second year Greek text. It's funny and thorough, combining wit with wisdom. If you're battling your way through level 2 Greek and fear you'll be swept away by a tidal wave of advanced grammar and syntax, ensure your footing is sure by delving into this delightful little book. Even if you're extremely well grounded in first year principles, you'll doubtless enjoy his amusing introductions and helpful illustrations. It's probably the only intermediate+ Greek text that I enjoyed simply reading through.
Another great intermediate work also worthy of shelf space is Richard Young's Intermediate New Testament Greek: A Linguistic and Exegetical Approach. (As an aside, I'd love to know what Greek font he uses - its the most attactive I have encountered).
Learn to Read New Testament Greek by David Alan Black
While I appreciate the inductive approach (recommended: John Dobson's Learn New Testament Greek, 3rd Ed) I prefer the traditional, step by step methods that have served students so well for so long. My favourites among the latter include the venerable Elements of NT Greek (recently updated and improved by Jeremy Duff) and the highly popular Basics of Biblical Greek Grammar by Bill Mounce (it seems to be the standard text in many US seminaries). But for me, my favourite introductory grammar is Black's Learn to Read New Testament Greek. Prof. Black has that most precious gift of saying a great deal with great simplicity without sacrificing elegance or clarity. That's enormously useful for a work of this type. Finally, Prof. Black generously offer users of his textbook a free pronunciation CD and workbook. Contact him via his blog for more details.
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One other little thing for absolute beginners: for a fun dip-your-toe-in-the-water inductive introductory starter, try Peter Jones' Learn Ancient Greek. It's even better than his highly rated Latin version. It's not restricted to Koine so expect to dabble with Plato alongside John's gospel but at the elementary level, it doesn't really matter. It's great fun, especially if you like British humour.