(photo of own copy, cover art by Raluca Marinescu)
Strictly speaking, the name is "Ulraunts Guide to the Planes: Acheron" but I think this block-buster tome needs no extra beefing up as part of a larger work. It stands tall on its own. As someone who came up through Planescape, the sheer size of this tome is jaw-dropping. Acheron in the Planes of Law was a booklet; jammed full of ideas, but even for Planescape it was a 32 page booklet plus 6 pages in the players guide of Planes of Law. Acheron got 4 pages in the 3.5e Manual of the Planes. In short, this is an order of magnitude more Acheron than we have had before.