Paul Kidd's Greyhawk trilogy of books (White Plume Mountain, Descent into the Depths of the Earth, and Queen of the Demonweb Pits) is probably the most entertaining books for D&D that I've read. However, for me a point against him is that both continuity of the Greyhawk world, and in how the rules work, often go out the window. There are even a few mild contradictions between books (namely the ending of book two compared to book three).
Prince of Lies, sequel to the Avatar Trilogy, by James Lowder, is a great book, showcasing a divine struggle very well for the Faerunian gods.
Troy Denning's sequel to that, Crucible: The Trial of Cyric the Mad, is even better. The character of Malik el Sami yn Nasser, who narrates the entire story, seems to leap off the page, and the characters all come across as very fully-developed.
Also by Troy Denning, the Prism Pentad series, set in the Dark Sun campaign, were quite good. The definitive books for that campaign, hands down.
James Lowder's Ravenloft novel, Knight of the Black Rose, is a classic, showcasing how the famed death knight, Lord Soth, entered the lands of Ravenloft, and became trapped there. The book does an excellent job of getting Soth's character down, being very insightful into his nature.
Likewise, the sequel to that, by James Lowder and Voronica Whitney-Robinson, Spectre of the Black Rose, is just as epic, detailing the long and painful series of events by which Lord Soth returns to the Dragonlance campaign.