What I'm trying to say, is what's going to make this different than a standard monster book outside of the theme of underdark monsters? ...
If you have any other questions, please contact Jim Butler." so I was trying to find out what excatly that meant.
Covering the same ground is hardly a new thing in this industry. For example:
Rules for ships/oceanfaring/combat: The Seafarer's Handbook, Broadsides, Seas of Blood.
Warcraft: Fields of Blood, Cry Havoc, The Open Mass Combat System.
Monsters: CC1, CC2, Monsternomicon, Minions: Fearsom Foes, Legions of Hell, Armies of the Abyss, Liber Bestarius.
Magic: Book of Eldritch Might I,II, & III, Spells and Spellcraft, Relics & Rituals I & II, Spells & Magic.
Scifi Spacecraft Rules: Dragonstar, Blood and Space.
Dungeons: Dungeons, Dungeoncraft, Into the Black.
To be completely honest, the only thing in my entire collection other than campaign setting material that seems completely original is Gary Gygax's Necropolis because it's an entire adventure that takes up a whole hardback book, and it gives us a great deal of material on an Egyptian type setting. What I find the most amusing about this is that regardless of how original it may be in relation to the rest of the D20 products, the material itself is more than 10 years old. I have the originals.
The precedent for this type of thing was set in the early days of D20. Personally I'll buy monster books until the Illithids come home as long as they keep making them and they keep being good. I'll also buy multiple books on other topics if I can get them to both work in my game and they both have something useful to contribute.