Hi Carl,
I'm seconding (thirding?) Colan's advice to make sure you're able to apply what you're learning. One thing I saw that stuck with me years ago from one of the info marketing spaces (might have been a guy teaching sales? Internet Marketing? I can't remember) but basically, for every hour you spend learning and reading, spend two in the field applying what you're working on. A second piece, if you're not actually going to apply something in the near future, it's not time to learn it yet. Learning something for 'when I need it' in this industry is a good way to accumulate outdated knowledge that goes obsolete before you've ever even done it.
I'll also second Colan's advice to start getting active in the official GMB community. I started doing that late last year, and it's been a huge boost to my knowledge and abilities, I'd highly recommend it. That and this forum has been the bulk of how I've beefed up my skillset in the last six months, the community here is awesome.
One book I would say to pick up though is
this one. It's short and entertaining, and doesn't necessarily hit best practices, but it gives some insight into what happens under the hood in a way I hadn't seen before. Really interesting, well worth a few hours of your time.
Last piece... the most important things you can learn in my view, is industry dependent. Best practices are important, but figuring out what's already working in your industry and keeping a finger on the pulse of the industry is huge. What kinds of backlinks make a big difference? Which citations are most important? What categories should you use on the GMB page? Which keywords are your 'money' keywords? What kind of copy/approach is going to convert? No book will teach you that stuff, but you can figure out how to answer some of those questions for yourself in these two awesome articles by Casey. Here's
one, here's the
other.
Ahref has a free trial, I like their backlink analysis tools. Places scout has a free trial too, and they're the only tool I've found so far that has the ability to pull together data on who's ranking where for what keywords... they're good for getting a bird's eye view. Those are my favorite paid research tools at the moment.