I recommend the above mentioned books in the other direction.
I prefer Pete Wolfinger's "Scuba Savvy" as it is an excellent and easy to understand primer on how regulators actually work, and while the examples and illustrations are of Scubapro regs for the most part (he was after all the reg tech training guru at Scubapro for a couple decades) the design principles transcend nearly all brands an models (expect some of the Poseidon second stages)
After doing reg tech stuff for 20 plus years I also firmly believe the right tool for the job makes the job a whole lot easier and also prevents mistakes or unintended damage to a regulator. It's not an argument for getting tools you'll never use, but it is a strong argument for getting the tools you not only need, but need to work more efficiently.
For example, you don't technically need an inline adjuster to adjust the orifice in a second stage. You can unscrew the hose off the second stage inlet and use screw driver to adjust the orifice incrementally then re-assemble and retest, then rinse and repeat several times until you get it just right. If you're a diver doing your own reg that's a workable strategy (although it needlessly increases wear on the inlet fitting o-ring, and increases the potential to pinch it in one of there-assemblies). However, if you're doing reg repair as a means of earning money, you need to be much faster and more efficient that that makes a good quality inline adjuster that can be quickly adapted to different second stages an indispensable tool.
There's a huge difference between buying a copy of a Harlow book and DIY'ing your own regs and becoming an economically viable reg tech, and having the proper tools is a big part of that difference.
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That said, it isn't something you can probably plan on making a lot of money doing. It can be a great part time job and a great way to make some extra money to support a diving habit, but making a living off it isn't what I'd call viable.
Shops generally pay tech two ways.
1) Under one model the tech gets 1/2 the labor charge per stage and that runs between $25 and $30 per stage, meaning a tech makes between $37.50 and $45 for a first, second, and octo reg set. If the tech is experience and very familiar with the reg, it's not hard to complete a service in about 60 minutes without skipping any steps. That can stretch to 90 minutes if the shop uses a bench test pre rebuild as well as a post rebuild bench test and flow test with documentation of both tests.
That's still $30 an hour and it sounds great, but more often than you'd like you get a badly cared for reg that requires extensive cleaning, that can easily double the time involved. Some shops add an additional charge for cleaning very dirty regs, but the tech still only gets half, and the charge is never enough to cover the additional time involved. From time to time you also get regs that have issues inn the form of least or non-spec performance that require you to tear one or more stages back down to trouble shoot the problem. That will also generally double the time required and cut your net hourly rate in half. You'll also end up doing things like tuning regs that have just been sold or just doing a flow test and condition inspection on a regulator that only requires an annual inspection and a full service every other year. The shop will charge maybe $10 for that, which means you'll do a thorough inspection, bench test, flow test and leak test and perhaps replace an o-ring or two and make all of $5 for 20-30 minutes work.
Now you're making closer to $15-$20 per hour on average. That still isn't bad, except few shops have enough volume in reg repair to keep a tech busy. You might see enough volume (6-7 regs a week) to keep you fully employed 1 day a week in a single shop that does a decent volume, and maybe 1 day every other week in the slow season. That means you need to be working with 2-3 shops in an area to be able to make any significant income, and that means you'll have uncompensated travel time and expenses traveling between shops. It also means you'll have to obtain and maintain certifications from multiple reg manufacturers, and you often end up paying some or all of the expenses to get those certifications.
And did I mention that you're self employed? You'll get a 1099 in the mail and you'll be paying your own employment taxes on that income, so it's not $15-20 per hour net, it's $15-20 per hour gross. Keep track of your receipts for those expenses, but be aware you won't be compensated for the accounting time you put in to your self employed business.
As I said, it can be a nice supplementary income, but you're not going to get rich off it.
2) The second model involves the shop hiring you as an hourly employee. The over all rate of pay is low, like most dive industry jobs, and you'll either have very sporadic hours, or you'll be doing other things in the shop. Inspecting task, rebuilding valves, and O2 cleaning task and valves is a given, but you'll probably also be spending tim on the sales floor. That's good thing for the customer as an experienced reg tech knows an awful lot about what makes a good diving regulator. Unfortunately, the designs you think highly of are sometimes not the designs the shop pushes the hardest (markup and margins vary a lot by company and even by model of regulator) or more often, they are higher end designs that are outside the customer's price range, so you'll have to balance your reg tech brain with your sales brain.
Once again, it tends to be a great part time job that can augment the funds you have available for diving - but if you're planning on paying your rent that way, you might want to come up with a better plan.
The downside of this model is that you might be expected to both repair regs and wait on customers who come in the door. That causes problems as when you get interrupted in the middle of a reg rebuild mistakes get made. And, if you're in the middle of a reg rebuild and you keep getting interrupted, your annoyance with the customers can start to bleed through no matter how well you think you're hiding it.
A variation on this model then is to just have a reg tech paid at an hourly rate who comes in the shop on a schedule and works on regs, tanks, etc. But again, unless the shop has a lot of volume, it'll be a very part time job and not something that will pay enough to make a living, and it generally precludes you from working for multiple shops.
