If I could have only one loading manual, I would get one from a manufacturer of reloading tools and equipment. The powder and bullet makers tend to recommend their own products rather than a competitor's. That's fine as long as you can get that manufacturer's powder or bullets where you are. The Lee guide is cheap and offers loads for a lot of different powders and bullets. Yeah, they tend to tout their own reloading gear, but you already have that so it will all apply specifically to you. Most people wind up with a multiple of books eventually. You can also often get load data from powder manufacturers from your powder supplier. Hodgedon has a pretty decent one on line, and my sporting goods store gave me free copy when I bought several pounds of powder. I look online to supplement my books. You will find that no two sources agree on maximum safe loads, play it safe starting out, and get a couple of recommendations before deciding on a starting load and working your way up. Most of the reloading advice I've seen here (including load data) has been conservative, safe, and decent. When in doubt ask here!
*Added: The little plastic bound books that are specific to a single caliber that you will see everywhere are a compilation of load data from dozens of resources, much of it I recognize as coming from my other loading manuals and component manufacturers. They are a good buy if you only plan to load for only a few specific calibers, and are a wealth of information for the specific caliber they are intended for.