All the Elric and Hawkmoon books are brilliant. Imaginative, thought-provoking, and mesmerising. The forthcoming titles, Elric Companion, Castle Brass, Cults, Magic... all will be paragons of excellence.
Okay, to be serious and less egocentric...
Simon's reviews speak for themselves. Charles Green has done brilliant work on Bright Shadows and Magic of the YK, and the Elric Companion takes a great deal of material that I couldn't fit into the main rules, plus a whole load of new stuff written for that project. Cults of the YK will expand cults further, but won't be a shopping list book of new and groovy cults; it will be a serious dip into who cults work in the YK, their relations, and what worshippers can expect.
Castle Brass is a detailed campaign setting and campaign outline for the Kamarg, Aigues Mortes and the Castle itself; lots of NPCs, maps of the regions, and lots of scheming and weirdness.
On Hawkmoon, I think Gareth Hanrahan has done a fine job. In particular, the opening of the Granbretan book is just fabulous, capturing the weirdness and comedy of the Hawkmoon setting in a really unique and imaginative way. A complete newcomer to the Hawkmoon saga can read that intro and know immediately what the flavour of the game (and books) is.
As Charles said, I'm very happy to talk about the Elric and Hawkmoon lines here, and on the Mongoose forums, so please feel free to ask questions.