Gormenghast I recently (finally) read the second book and I'm now watching again in youtube the miniseries that made such impression on me when I saw it on TV around 15 years ago.
As in pretty much everything the books are better, but I still name this miniseries pretty much the best live action fantasy in any format I've ever seen.
The series is curious combination of being in some part more childish and comedic than the dark and dry humor of the books ever, but other times as deep and grim as the original. As if makers couldn't decide if this is for children or adults.
Nevertheless that effect fits well to the material.
About the books I must say one thing, especially if you're not native English speaker: the language is extremely difficult. I've read in English Hitchhikers Guide to the Galaxy series, I've read LotR and number of other stuff, but I'd say Gormenghast was the hardest I know.
But at the same time, they definitely are among very best books I've read and that's particularly due characters. While some are caricatures to add little humor to the gloomy world, many are deep and develop in unexpected ways.
Here's the first episode, there's four one hour episodes.
(Series follows first two books, "Titus Groan" and "Gormenghast".)