Re: How to improve the left hand.
A difference in ability between hands is not much of a problem. Habits which magnify that difference over time are a concern. I think that is exactly what you are asking about.
You will definitely benefit from suggestions for work from Stick Control. As others have said, make sure you dedicate at least as much time to exercises with left hand lead as you do to the ones with right hand lead. You probably have a big enough gap to warrant practice time specifically to catch up. Just remember, that a single crash course won't lead to steady and ongoing improvement of your left hand.
A practice routine which engages and challenges both hands is a great way to address the shortcomings of the weaker hand. As others have suggested, practicing new rudiments and associated exercises will be great with this. I'd take it a step farther and look for a book with rudimental snare drum solos or marching band street beats. Two-thirds of the work might be getting your left hand up to speed, but it will feel more like a single challenge to both of them.
You can apply similar reasoning to the drum set. Find some new material you have not played before. Then try to play it both with left on a cymbal and right on snare and vice versa. Some books, like The New Breed (Gary Chester) already make you do this.
After starting primarily on the drum set, I spent several years playing a lot of keyboard (marimba, xylophone, etc.) percussion. That really helped me because many melodic lines are naturally easier to play with a left hand lead.
Finally, you could also look at Accents and Rebounds for the drummer. It's the sequel to Stick Control.It is very difficult, so I wouldn't recommend diving in right away. It is a nice progression from Stick Control and I wish I had tried it sooner. Just be warned that it's really tough. It is to Stick Control what the Japanese Super Mario 2 (Lost Levels) is to the original Super Mario Brothers.