One thing to keep in mind is that this book is less a "book" than an anthology of essays. Essays from different writers, that were apparently all given the same narrow topic, and who all wrote around the same talking points. I feel like I'd have gotten the same thing out of it by just reading one of the essays (the last one, given a choice) as I did from reading the book as a whole.
The writing is, as has been mentioned elsewhere, uniformly poor. Each essay reads like a transcript of someone asked to speak extemporaneously about what Luciferianism is, with no editing done at all, was cut and pasted into the submission field. As a result the essays wander back and forth topically, the spelling is atrocious, and the grammar is, if possible, worse than the spelling. Sentences start on one topic, then mid-sentence change topic as though the writer forgot what they were talking about. These are not long sentences that they're getting lost in. There's at least one paragraph whose first sentence starts halfway in, as though they were copying it from elsewhere, and didn't manage to select the first half of the sentence.
To say I had a hard time reading it due to these structural problems would be an understatement. This may not be as big of a problem for you, but if badly structured writing bothers you.... just be forewarned.
As to the subject matter. This is an introductory text, and as such is a bit on the light side. The essays give a brief overview of what the GCOL believes, but never seem to get past that into any actual beliefs or practices. I suspect that's intentional as they sell several grimmoires (at several hundred dollars a piece), which likely contain that information. Several of the essays are explicit in saying that they don't believe that these gods are extant beings, but rather that they use the names and their associated meanings (to the original cultures, as well as to themselves) as a jumping off point for accessing those traits in themselves. These same essays will in other places abandon that premise, though whether for ease of writing or because it is just a premise is hard to say. Other essays do away with it entirely, speaking as though there's an extant being that they are trying to commune with. Again, I think this is an issue of poor writing, rather than an actual tennet of their philosophy, but that's supposition on my part that I can't confirm from what has been written here.
I still think it's probably worth a skim, especially as it's free on Kindle Unlimited right now, but I wouldn't say it's worth any kind of deep dive or worth adding to your library in a permanent fashion.