Here is a great grammar book by Annette Lyon called There, Their, They're A No-Tears Guide To Grammar From The Word Nerd. I read the entire book in about 3 hours in between motivating my kids to work. It has a lot of great helps on how to improve your grammar and for the low price was well worth the investment.
I think that what has really interested me the most about learning grammar is the way it can be used to make a good story even better.
I highly recommend that everyone take a look at the book.