I can recommend a couple of different programs out there. I know its a sea of different ebooks out there claiming to lower your score. Let me narrow it down for you.

Ultimate Short Game System
Cody Wheeler is an experienced golfer who goes into a lot of depth teaching the short game. Yeah it includes chipping, and sand shots, but he spends a lot of time on putting. I mean putting is part of the short game, right? Just consider the rest a bonus because for a reasonable price he gives a wealth of information on all aspects of the short game including putting.

Master Your Putt
Jim Goldstein puts together a pretty comprehensive putting instruction ebook here. He is only focused on putting so if you don't want to spend money on your whole short game in Ultimate Short Game System, This is as good as it gets for putting instrucion. Highly recommended.

Mind Power Breakthrough Mental Game Coaching
Wade Pearse offers a great system for conquering the most important part of putting, and golf in general. You have to get your mind in the right place to succeed at golf. It is the biggest barrier to playing great golf. I highly recommend this ebook as it will truly help you lower your score. I cannot stress enough how important this is. Don't neglect the mental aspect of golf, it is the hardest to master, but will improve your score the most.