The most disappointing book of the past month or so was J.M.Coatzee's Disgrace. It was hugely hyped in the Booker, but - for me - it was a tawdry book which succeeded only in making me detest both central characters to the extent that I wished them a painful and bloody death within the first 30 pages!
As for the worst book ever written - hmm....
But, at the risk of sticking my neck so far out that I'm crusing for a bruising, I'd suggest the Bible, insofaras more people have been killed as a result of its misuse than any other book (though Kapital and the Thoughts of Chairman Mao could probably run it a close second)
Failing that, anything published by the 'Church' of Scientology or that old fraud and fantasist L.Ron Hubbard.