I am interested in becoming a k9 handler, and I'm in the very beginning of my career. I am taking the advice of guys on here as well as k9 unit guys I have met and taking things slow because I know it will take years probably before I ever get the opportunity to put in for the k9 unit. In the mean time I am looking for books that could enhance my knowledge of k9 training, or anything else that will make me a better candidate if that day ever comes.
Asked a few of my dept buddies who are K9 and the general consensus was don't worry so much about the K9 training as your agency will teach you how to handle the dog the way they want it to be handled (general care, play time, training exercises etc). Their main words of advice were to be the best deputy/officer you can be handle calls/be self reliant plus be in good shape.
I'm picking up my dog this week actually.
I will say from my humble experience, K9 (at least where I'm at) is a pretty exclusive group. I got into the suit every time I could and I trained with them nonstop, even as a fresh faced rook. The guys that will vouch for you want to see if you're dedicated to the program as a whole, not just as a handler. If it means getting junk punched by a Mal then do it. They'll remember that when a handler slot opens up.
I would advise looking into K9 first aid and how the olfactory system works on a dog. It's pretty neat.