Warriors of Heaven
Warriors of Heaven is a 1999 splatbook for Advanced Dungeons & Dragons 2nd edition. Like its counterpart, Guide to Hell, it is a plane-focused splatbook that, for some insane reason, wasn't marketed with the Planescape brand, causing it to be mostly overlooked.
This book is a player-centric expansion upon the Upper Planes and the angels that dwell there, providing new magic spells for priests and wizards, expanded aasimar material, and not only expanded lore, but also full PC writeups, for the full range of Celestial creatures! That's right, this book lets you play an Aasimon, Archon, Asura, Eladrin or Guardinal!
One unusual element of lore introduced in Warriors of Heaven is that only Aasimons, Archons and Asuras are created from the souls of Upper Planar Petitioners; Eladrin and Guardinals, despite often being considered the ascended souls of elves and animals, are actually entirely seperate species who dwell on the Upper Planes, marry, have children, and ultimately die, just like mortals do!