So what sources would you suggest for my inquiry?
In general, *any* "Let's be an encyclopedia of dozens of different topics" is going to be a bad choice for learning much about any specific topic, and particularly something like deities.
That goes double or triple for something like deity information: no one person is going to be expertly familiar with dozen of different cultures and their related materials, so at best you're going to get brief summaries, and at worst you're going to get brief basically useless summaries. (And it's usually the latter.)
Encyclopedias are good for a general overview, and for giving you terms and other details that will let you do more research. (Things like dates, place names, etc.) In most cases, Wikipedia's going to be a better starting place for that than a published encyclopedia by a single author. (Especially since for a lot of mythology topics, Wikipedia makes it fairly easy to click through to other deities in that culture, or other background material.)
For actual learning about deity, my usual advice is to look for books about a given particular culture, and ideally a couple of different ones including at least one from a more academic perspective, because those will often lay out more about where the information comes from and put it in a larger historical context.
There's a series of books in the Legendary Past series, put out by the University of Texas, that are a couple of decades old at this point (so you need to be aware that people have found more information and learned more things and come to new conclusions since then) but that have well-done illustrations of artifacts and inscriptions and whatever else is relevant.
This is the link to the Celtic myth one, for example, and the others in the series should come up with that.
As an example of different places to explore, I also like the approach in the book
Devoted to You, a series of four long essays about deities edited by Judy Harrow. Four different people talked about deities they honoured and had done extensive work with, and also talked about the kinds of resources they found useful.
Again, it's an older book (came out in 2003) so some of the resources mentioned, especially on the web, are no longer going to be active, but each of the people writing talks a lot about their learning process, and you can use the process to find newer materials.