Well to build explosive muscle, your going to want to build up your Type 2 B fibers, or your fast twitch muscle fibers. These fibers respond to heavy, low rep work that will extend for no more than a minute in length, and doesn't require the muscle to start oxidating itself. So for baseball players, biologically, you want a workout that emphasizes fast twitch work on the upper body, and an equal split of fast, med-fast, and slow twitch on the bottom. To do this, I reccomend a few things. For pitchers, stick to heavy hard sets of 3-6 reps for upper body, with 1-2 minutes of rest inbetween sets to allow your muscles to remake some ATP, and lasting for about 4-5 sets. However, because heavy sets like this can result in injury if optimal form isn't used, I reccomend that you use mostly machines for upper body, and take your sets reps slow and controlled. For Lower body, a mix of 3-6 rep range, and 8-10 rep range can be used, and both are useful in baseball terms, and long distance running can help build your slow twitch fibers. And the number one rule to remember in my mind when training like this, is stretch A TON. Building up the muscle will mean nothing if you lose your flexibility, you need to be able to recruit more muscle than normal in your natural motion to see velocity gains. Recruiting more muscle in a shortened delivery will be a tradeoff and in the end useless, so please make sure you get atleast 5 minutes of stretching in before a workout, and 10-15 minutes after. This will give your muscles the length and flexibilty they need to preform baseball related movements.
Just my 2 cents, its how I've been working out this offseason and I've seen a definite increase in velocity, command, and endurance.