I'd like to update my last post with a few bits of information/corrections.

1. Apparently scannerdanner actually does have a book as well that seems to be very well organized to cover all of the different types of testing procedures. It appears to delve deeply into the fundamentals and theory as well as showing you the actual real world application of that theory. That's probably exactly the kind of thing you're looking for. It will give you a full education in the material as opposed to just teaching you how to do a few things, and that is very valuable. I think it sells for like $50 which isn't too bad... It's a bit pricey in the age of getting everything for free online, but I think it's probably worth it. There is also the video lecture series available which kind of augments everything covered in the book. I believe he teaches at a technical college in PA.

2. If you don't yet have a DMM, then I might suggest trying to find a good deal on a DC clamp meter. That way you're getting both an ammeter and DMM in one instead of having to buy, and use, 2 separate tools. The good DMM's can usually measure 10A DC, some can even measure 20A DC, but that isn't a lot of current. Also, those limits are usually only for a few seconds, and need to be followed by a 10 minute or longer period of rest to cool down if you're close to the limits of the current. 10A often won't even be enough to measure the current draw in the power distribution lines out of the fuse box. Something as simple as checking for parasitic draw could blow the fuse in your meter, and you can absolutely forget about measuring the current to the starter. Ideally, if want to have something that could measure up to say 400A. That should be enough to measure most starter circuits. If it's not, then there is a good chance there is something wrong with the circuit you're testing. It should average about 300-400A in most vehicles.

3. If money were no issue, then my ideal equipment would include what I'd describe as a super test light on steroids the Power Probe IV. Look it up... It's got all kinds of nice functions; A good DC clamp meter capable of at least 400A DC; a bi-directional scan tool like the autel or Snap-On if you're working on 96 and newer vehicles (if you get something like that, then you might want to get probes that connect to it instead of independent DMM's and clamps. They have oscilloscope features when used with the scan tool); and there is a really cool tool that ScannerDanner introduced me to called the...uactivate assistant... Something like that. It basically let's you plug it in to a relay socket, and you can test both load and control sides of the circuit. It even has a loop to hook up a clamp meter along with 4 sockets to connect your DMM probes. The problem is that it is $200, and I think that is absurd for what is basically a case, switch, connectors, and a pair of LED's. You could, however, easily make your own dirt cheap, and I think you could even make a better version. This version only tests 4pin relays. What about 5pin or more relays? How about that 9-PIN OVP relay? You could absolutely add that functionality in your own creation without too much difficulty. You could add in an extra wire that runs between the battery + terminal and another switch on the device that would let you simulate turning on the key for example. Some relays have that additional switched 12V+ pin, and it really would be easy to add. I'd bet that you could buy all the parts, including a nice weather resistant project case, for under $20, and build it in under an hour with so the head scratching and soldering time included. Heck, you could rig something together for about $5-$10 I bet in about 5-10 minutes if you wanted to. You could probably even add in relay testing capabilities as well (this device currently tests the circuit, not the relay itself).

TLDR- SCANNERDANNER does offer a pretty good book apparently in both digital and hard copy. I really like the Power Probe IV. A good DC clamp meter can kill 2 birds with 1 stone. And you can build your own version of the $200 "uActivate" relay circuit tester both cheaply and easily.