Gene Wolfe is often, really quite often, called The Greatest. I agree, it's deserved. He's most famous for three tremendous series:
The Book of the New Sun and its sequelae; the Book of the Long Sun; and the Latro series, starting with "Soldier in the Mist".
That should be enough for anybody, but some reviewers are complaining that this isn't a monumental series like the New Sun.
GW is also an extremely idiosyncratic, difficult writer. I loved the above mentioned series, but a lot of his other works, I can't get into.
I had trouble getting started on this one, but once I did, I really enjoyed it. And no, it's not the New Sun, but I don't think that means there's anything wrong with it, or his "powers are declining", or anything like that. And btw, so what if his powers ARE declining?
"When nine hundred years YOU reach, look this good you will not!" Somebody who can write this book, imho, still has a lot on the ball. If you want him to write another giant series, maybe you're being just a little too demanding.
I'm not going to recap anything; others have done that, and said a lot of interesting things too. I just want to say, for the newcomer to GW, this might not be the place to start. I'd recommend starting with any one of the three big series, most of all New Sun (first volume is called Shadow of the Torturer).
For GW fans, I'd say, look, I think GW is the greatest, and I don't like, or can't relate to, some of his books. There may be some I just haven't managed to get into yet. All his books are weird, if you like; all the stand-alone books different, though they may all be the same in some ways. I like this one a lot, and recommend it.