Of the series as a whole, I think Hordes of the Abyss is my favorite. It's extremely well done, the subject matter of the book is very iconic and interesting, and it's engagingly written and the ideas spark the imagination. It's diabolic counterpart is also good, although Hell as corporate cubicle culture was either an extended joke that went on too long, or just plain not that great an idea in the first place. But in general, the series quality overall has been excellent, and I have little but good to say about Lords of Madness and The Draconomicon too. Which is why Libris Mortis is so disappointing; it's not that it's a bad book by any means, it just feels very pedestrian and boring compared to its collegues in the monster speciality subseries.
In any case, I've moved on to Monster Manual V for now. I think my next gaming book forays may be into some Paizo material that I've had for some time, but not read because the format is so difficult for me to work with. I'm actually going to buy some hardcopies of books I already own on pdf, and I've learned my lesson not to go that route anymore thinking that it's a time saver and a convenience. Clearly, at least for me, it has not been. Maybe if I had a laptop it would be different. And maybe in the next year or two I will have one. But for now... it's been difficult to read pdf books, and I've made very little headway on books that I've owned, literally, for years in some cases.