The bullet manufacturers provide test data that they have actually gathered and the loading range is usually based upon the max powder capacity of a cartridge with their bullets up to the SAAMI limit for pressure.
I find the Sierra 5th edition manual the most helpful and complete but also have Nosler, Hornady, Speer, and the new Berger manuals as well. After gathering all those manuals, I find that I use the Sierra Manual all the time for the general load range information and use the other bullet manufacturer manuals when I happen to choose a new bullet to see the recommended seating depth and whether the range for my favorite powder is the same.
The Sierra manual shows the load data for each bullet by powder and has the load for increments of 50 fps, usually, so you get about 6 to 8 loads for a particular powder. I find that that comes in very handy when you are working up a load with a new rifle and want to try a range of velocities to see where the nodes are with a particular bullet weight.
Most of the other manuals provide only the low end, the high end and a mid load. Each manufacturer has their own way of presenting the data on their bullets and often they don't agree on the load data for a particular bullet weight and powder, possibly because they used different barrels or primers. Based upon results, I have found that the Hornady manual is very conservative in their load ranges - maybe it is a lawyer thing.
I also get powder load information from the web pages of the powder manufacturers and match it to the bullet manufacturer manuals to see if there are any anomalies.
You could save on the cost of bullet manuals and get the bullet loading data from the bullet manufacturer's web page also but you would wind up spending lots of time to compile it. If you only load for one or two bullets, then it might be the cheapest solution if all you want is the loading range.
Many of the bullet manufacturer manuals tell you what barrel length that was used in their actual tests.
Knowing the test barrel length to compare to your barrel length is important if you are trying to match velocities to your chronograph results in order to come close to a velocity that your rifle seemed to like with another bullet of the same weight and shape.
Lee and Lyman appear to me to be compilations of several bullet and powder manuals. I have found that their data is sometimes considerably different than the data in the latest edition powder manuals. It might be that they are keeping old data in the manual from previous powder recipes and don't update it when the manufacturer issues newer results.