It should be pointed out that this book is written as an 'alternate history', detailing a richly imagined and meticulously researched tapestry of Nazi plans, activities, artifacts and personas.
With that out of the way, this is a must-have for GMs who are planning to run games (or are already running ones) that are set in WWII with a touch of mystical / magical elements, weird science hints, or even Cthulhoid intrusions. It's also a treasure trove of origin hooks for superheroes or supervillains from the WWII / post-WWII era of comic book gaming.
Even a cursory read will clue you into names major and minor in the network of allied and opposed occult factions; a close read will give you a rundown of a multitude of Ahnenerbe activities around the world, major artifacts that passed through Nazi Occult hands like the Spear of Destiny, the Holy Grsail, the Ark of the Covenant (with a tip of the hat to our favorite adventuring archaeologist), Nazi mystical rites with occult significance with their exact dates in the timeline, and ties to locations like Agartha and to technologies like the Vril-powered Bell.
If you want to freak your players out with fantastic and horrific adventure elements strongly grounded in history, this is the book for you.