Many that I have met who have not read a reloading manual think it is a book full of reloading recipes. Some are - like pamphlets - using just one brand of powder or one brand of bullets. Some that were sold are repeats or re-prints of some other company's previously pressure tested recipes. Many will have 25% to 50% of the book with articles and chapters about how to do reloading, using various brands of tooling, about internal and external ballistics, and so on. Many have gone through multiple editions over the years - I notice most of them have a statement that they dis-avow anything written in their previous editions. Sometimes bullets, powders or cartridges are added in newer editions, sometimes some get dropped. I have many older ones - out-of date, I guess - but with data for older cartridges that I have like 7x61 Sharpe and Hart or 308 Norma Mag - many times cartridges like that are no longer listed in the "newest" manuals.
And some of us have found typo's or errors in both the printed version and the on-line version - so I no longer rely on just one source to suggest my next loading - I much prefer to cross check with various sources - none tend to be exactly the same to each other - is part of this game to understand why that happens - and should maybe suggest why your rifle might not be identical to the one they did their testing with - getting different results. And then I typically "work up" my loadings with new-to-me components in small increments - I no longer enjoy "surprises". With very few exceptions - like one or two - I do not use other people's hand loads or recipes - certainly none of them without each component identified - I prefer to do my own "work up" in my own gear - and no one had my rifle to come up with loadings in the past.
I suppose the first "manual" that I used was the card that came in the Lee Loader reloading kit that I started with - late 1970's. Pretty much monthly, since then, I would get various magazines like Guns & Ammo, Shooting Times, Handloader, Rifle and so on. I also have several books by writers like John Barsness - often those "books" are collections of articles that they previously had published in various magazines - what they do for a living, I think. I do recall loading from Nosler No. 1 Manual and Sierra No. 1 manual - when they were "new". On my desk right now - that I refer to some most weeks - Sierra Edition V, Lyman 50th, Western Powders Edition 5.0, Hodgdon's 2013 Annual Manual, Woodleigh Loading Manual, Hornady 9th, Speer #14, Nosler #9, Modern Reloading Second Edition by Richard Lee, VihtaVuori Oy 4th Edition, Winchester Powder Loading Data 2nd Edition. Some were free - at retailers or from on-line downloads. Many are "out of date" - is newer editions available. Most all reloading tools that I have bought had manufacturer instructions how to set up and to use their devices - not all the same, for sure - and is many "kinks" and alternate uses that have been published by others - to use those devices in ways the maker had not identified.
Several of the manuals will tell you how to do stuff - get a result - that someone else will sell you a device or devices of their manufacture, to buy from them, to accomplish the same thing. Finding the lands in your rifle and establishing a Cartridge Over All Length is an example - Woodleigh Manual tells you how to do that using your cleaning rod and a dowel - I have read the similar process in other sources - or some companies will sell you some tooling to get the same result. Up to the re-loader, I guess - some prefer to read and learn and maybe "make do"; some prefer to spend money and accumulate "stuff".
I can not say there is one "best" manual to get - each seems to add a bit more to the learning. Typically the Lyman, Nosler and Sierra Manuals have a lot of "good stuff" in them, besides loading recipes. So does Woodleigh, Hornady and Speer - and so on. Sometimes it is "comforting" to read similar stuff in multiple sources, sometimes one source will wander into areas the others do not - Richard Lee's discussion and ideas about lead cast bullet hardness versus breech pressure, for example - not seeing that repeated in many other sources.