The manual you need for planar monsters.
This is an excellent resource that includes many of the important planar creatures that didn't make it into the Monster Manual, in particular most of the remaining planar Exemplars, such as the full suite of Archons, Guardinals, Eldrins, Rilmani, and Demodands/Gehreleths.
It also includes more Yugoloths, which got a minimal presence in the MM, as well as Hierarch Modrons, Hordelings, and much more.
The monsters themselves are converted with proper 5e philosophy in mind. Rather than attempting to reproduce every feature, the aesthetic and essence of the creatures was retained, and the most distinctive elements were translated—making the monsters inside feel like they could have been in the Monster Manual. This is an undervalued talent which makes these conversions stand out as superb.
The only downside is that there aren't even more converted monsters.
Whether you are a fan of the rich 1e-2e planar cosmology, or a newcomer to D&D with an interest in the wondrous planes of the multiverse, this product, in concert with the official Monster Manual, will give you what you need to flesh them out into real locales.
This product is worth the price.
[5 of 5 Stars!]