I know there are several books which compare Ancient Greek with modern like Jannaris' Historical Grammar. But for modern Greek (including demotic) there is Greek: A Comprehensive Grammar of the Modern Language (Comprehensive Grammars) by David Holton and Vassilios Spyropoulos (Paperback - Mar 15, 2012) http://www.amazon.com/Greek-Comprehensive-Grammar-Language-Grammars/dp/041559202X/ref=sr_1_3?ie=UTF8&qid=1333935532&sr=8-3
. It's a very thorough modern grammar.
But I just picked up Essential Modern Greek Grammar (Dover Language Guides Essential Grammar) by Douglas Q. Adams (Paperback - Sep 1, 1987) http://www.amazon.com/Essential-Modern-Grammar-Language-Guides/dp/0486251330/ref=sr_1_1?ie=UTF8&qid=1333935623&sr=8-1
. Essential Modern Greek Grammar is just what is needed for a student of Koine to get a quick look at modern Greek. The languages look very similar, once you know that a lot of beginning vowels have fallen away. This book is 90 pages long. Super cheap, and should be read by those who want to have a grasp of the difference between modern Greek and Koine.