As you'd expect from a collection of four novels written over a period of twenty years or more, this is uneven. The first two novels hand together quite well. In the first, Enderby tries to avoid engaging with the outside world; in the second, he is forced to do so. As with anything by Burgess, it's funny, wordy, and vocabulary-expanding. Enderby himself is a very ambiguous fellow. He's a truly horrific human being, but also strangely appealing to anyone who has elitist/artistic tendencies.
The last two novels are a different matter. They don't hang together, either with each other or with the first two. The Clockwork Testament is the worst novel of the four, in terms of art (less well written, less unified) and morality (most contemporary readers will judge at least Enderby and possibly also Burgess to be racist) but also the most interesting in terms of literary history and philosophy: it is, quite obviously, a response to the American reception of A Clockwork Orange/i>. This is perfectly reasonable in purely intellectual terms. The novel was butchered by the film version, and Burgess was made to appear someone he really isn't (i.e., the film is a celebration of the individual freedoms so beloved by seventies radicals; the book is actually a rejection of the juvenile antics that bear some resemblance to the antics of seventies radicals); here, Enderby spends much time thinking about Dostoevskian themes of individual freedom, determinism, and predestination, in a fairly responsible manner, i.e., in exactly the manner that the film of ACO, and the aforementioned radicals, refused.
Unfortunately, Burgess lets the radicals themselves into the book, and promptly becomes the kind of reductionist that he claims those radicals were. Racism is mostly in your head, and so is sexism, etc... No, they're not.
The final novel, Enderby's Dark Lady, could almost be an apology for writing the Testament. The 'dark lady' of Shakespeare becomes a wonderful black American woman, who is far more intelligent, beautiful, sensible, and personable than Enderby... and also clearly a victim of blatant sexism and racism, which is not, as it was in the Testament, excused. It's also a very funny piece of anti-Americanism, if you like that kind of thing. And let's be honest, you got to the end of a review of a series of Anthony Burgess novels, so you probably do.
Unfortunately, the last chapter of this last Enderby novel is very, very silly.