I read these in school (a very long time ago), before I ever saw the films themselves.
Frank Chandler was a pseudonym of the British writer Terry Harknett, and this was his first western. As "George G Gilman" he went on to write the epic pulp western series "Edge", more vivid than his Fistful of Dollars novelisation, and considerably more graphic. As "William Terry", he also wrote the novelisation of the Anglo-Spanish western A Town Called Bastard, which actually outdoes the film in terms of violence and sadism, and captures the grotesqueness of a good spaghetti western more than anything else I've read. If you want the literary equivalent of a film like Blindman, then this is the book for you!
Joe Millard's versions of For a Few Dollars More and The Good, The Bad & The Ugly are straightforward and enjoyable - particularly the latter, which was based on the original Italian script and included scenes which weren't in the original English version. I couldn't stand his unofficial sequels, though - they thoroughly unravel the Man with No Name's mystique, the action is tame and the stories are just silly.
The only 'sequel' worth reading is A Dollar to Die For by Brian Fox, which reunites the Man with No Name and Tuco in a cat and mouse chase through post-Maximillian Mexico. It's certainly the closest in spirit to the SW genre, and probably the best written of all of them.