I don't mind supporting Obsidian by paying extra, but 20 bucks more for the Overlord Edition compared to the Archon Edition seems a bit pricey, considering all I get for that is basically 2 PDFs.
Are the Art Book and the Collector’s Guide Book worth the extra cost? How much content is in those books? I couldn't find any detailed information about them anywhere.

Doesn't matter anymore. Since it's on Sale now I got the Overlord Edition for the price of the Archon Edition.

These are SUCKERS editions any way you cut it. It used to be that only big studios were greedy, now big and small companies engage in these questionable crash grab for minimal efforts, all for some extras you only look at for 2 seconds and never look at again. The only "special" edition I would get is one with the main game plus all the expansions.

These companies need to learn how Criterion does it. Criterion is a DVD/Blu-ray publisher that pioneered way back in the 80s the so-called "special editions" that offered not only the main item, but also extra bonuses. The difference is that Criterion never charges more for the bonuses. They always release one edition only with all the extras. Nowadays we see greedy movie companies release "Steelbook editions" and what have you just to charge a higher price. But Criterion never does that.