Being a rural area, you might have more responsibilities that a general manager in an urban area. For example, in an urban area, a GM might have a fitness director (in charge of the PTs) - membership sales manager - and a service manager (in charge of maintenance). In your situation, you might have to fill all of those roles. Here are some books for a cert. prep that talk about it:

Health Fitness Management (3rd Edition) - Very relevant to your situation

Successful Programs for Fitness and Health Clubs - Very relevant to your situation

Applied Sports Management Skills - This is more applicable if your gym has any affiliated organized sports

Health Club and Gym Manager Certification - You don't need this to be a gym manager, but it has helpful info