Owner's Manual/Helm's Service Manual
We know what you're thinking. "I've had my owner's manual in my car since the day I got it." Well, good for you, but do you ever actually use it? As much as we might hate to admit it, we're not all mechanical geniuses, and there are going to be some problems we simply don't know how to handle. That owner's manual will fill in the gaps, and there's no shame in admitting it. It'll show you all the basics, like what type of oil and gas to use, tire pressure, liquids capacities, and all that jazz.
In addition to that, keep a service manual with you, as well. If you're in a hurry or if you're just in a situation that you quickly want to get out of, being able to figure out what the problem is the first step to getting back on the road.