So my favorite 3/3.5 book was the Fiendish Codex II: Tyrants of the Nine Hells, and this guidebook is the perfect spiritual successor. With an updated history and politics which made Codex II such a genuinely fun read.
I love that McGovern has taken the details given in previous books (both 5e and earlier) and expanded it to the most comprehensive guide to one of the most interesting places in the D&D multiverse!
I was also so happy to read information on Archduchess Zariel, I have been utterly fascinated with this character since I first heard mention of her, and I am happy to report she is just as wicked and awesome as I could have dreamed.
It's a perfect book if you love Fiends as much as I do and you aren't turned away by spending 15 bucks.