Dungeon design guides are fairly common, but most of them dwell on minutae already covered in Dungeon Master's Guides. This product is especially useful because it ignores most of that stuff in favor of things that haven't been adequately covered. It also presents some easy organization techniques to manage information overload and streamline the dungeon creation process. Some highlights of the product are:
-Advice on making semi-realistic caves, tombs, castles, and manor houses. "Semi-realistic" is explicitly stated as the goal here, as opposed to completely abstract or restrictively realistic designs.
-Random inspiration and idea tables that are both evocative and flexible. Most random tables don't work well for me because they either assume a very specific setting, contain a bunch of really bland concepts, or both. But this book has long tables with lots of genuinely creative ideas.
-A streamlined method for figuring out the dungeon's history and construction. I usually minimize or avoid this issue because most products make dungeon histories sound dreary and irrelevant, but this book's method is faster and more interesting and changed my mind on the subject.
-A streamlined sector-based method for laying out dungeon levels. The writer suggests going back and doing the typical intricate graph paper designs afterwards, and gives a little advice on that, but the book doesn't really dwell on that and works well for those of us who prefer simpler flowchart dungeons.
-Overall, almost everything in the book seems designed to make dungeon creation more efficient and less intimidating, rather than simply introducing more complications than I'd have if I did it by hand.
The art is mostly classic public domain stuff, but it looks pretty good in this context. The layout is decent enough and works especially well on a tablet, since it's single-column and uses a large font. The only downside is that the index is located 11 pages in and takes up several pages, and there's no bookmarks. Furthermore, as of this writing, the PDF is locked down so that new bookmarks can't be added, though that might be changed in a later version.
As an aside, this is an OSR product, but doesn't have two of the more infuriating traits that some OSR products import from the worst products of the old days.
One, it doesn't have a bunch of stupid puns and "hur hur this game is so silly isn't it" stuff a la Mystara or some Greyhawk products; it takes the subject matter reasonably seriously. If you're looking for a more serious OSR product, this is a great one.
Two, it doesn't have big issues with sexism or racism. There's only two problems that I found; one is that it mentions "African" under "primitive themes" on one table, and another is that it occasionally uses "gypsies" as a concept, though it's generally much better than most products on that subject. Fair warning there, but it's pretty solid and comfortable on the subject. It's overall better than the average TSR or Wizards of the Coast product when it comes to the subject.
In conclusion, I've found this product very inspiring and useful, so I'm giving it 5 out of 5.