As C.S. Lewis famously wrote, "If we have to choose, it is always better to read L. Ron Hubbard again than to read a new criticism of him." Actually he said it about Chaucer, but it's an interesting general principle. Sometimes I agree. At other times I prefer a good critical book about science fiction to actual SF, especially if the latter is by L. Ron Hubbard.
Now I don't mean those dire academic volumes with titles like Some Lesser-Known Aspects of Eighteenth-Century Utopian Fabulation in Albania. The great SF/fantasy critics are mostly practising writers who praise stories from an interestingly original angle or put the boot in with joyful style and elegance – like Damon Knight with In Search of Wonder (1956, but look for the expanded third edition of 1996), Kingsley Amis long before his knighthood with New Maps of Hell (1960), James Blish with The Issue at Hand (1964), Ursula K. Le Guin with The Language of the Night (1979), Algis Budrys with Benchmarks: Galaxy Bookshelf (1985) or John Clute with Strokes (1988).
The SF criticism in my own home library fills thirteen feet of shelves, so I could carry on listing titles for ages. However, the long succession of names might become a little too like Beachcomber's vital but fortunately imaginary reference work The Anthology of Huntingdonshire Cabmen. In these degenerate latter days, there are even several critical collections by me, of which it has often been said ... but never proved.
Here are some recommendations from recent reading, one dated 1987 and three from 2014:
Robert Silverberg's Worlds of Wonder (the 1987 title) is cunningly disguised as an anthology of classic SF: I didn't buy it when it appeared because I knew all the stories, some by heart. Silly me. What I missed is that each tale comes with an essay from master craftsman Silverberg, taking it apart to show just what makes it a classic. His revealing autobiographical introduction "The Making of a Science-Fiction Writer" is also a must-read.
Similarly, Jo Walton revisits nearly 130 old favourites and ponders why she loves or no longer loves them in What Makes This Book So Great, a selection of her many hundreds of thoughtful posts in this vein from the Tor.com blog. She usually has something wise to say. I nodded often – in agreement, I mean, not nodding off – and only rarely shook my head. Her tactful meditation on SF Series That Went Downhill is full of sad truths.
John Clute's latest nonfiction collection – there have been several since that 1988 debut – is called Stay and as usual throws you in at the deep end of a deep mind fond of occasional "studiously flamboyant obscurities" ... to quote his SF Encyclopedia entry, written by one John Clute, who should know. Besides many densely meaty reviews, Stay includes five short stories (where else could you find an image like "an entablature of salamanders loosed suddenly into a myoclonic can-can"?) and his 2006 mini-encyclopedia The Darkening Garden: A Short Lexicon of Horror. Whose approach to horror is like no other.
Adam Roberts – known to Princess Bride buffs as the Dread Punster Roberts – publishes witty, learned criticism at a great rate on his blog Sibilant Fricative. For reasons which are deeply unclear, Sibilant Fricative the book collects material not from its namesake but from his now-deleted former blog Punkadiddle. Besides being insightful, these reviews include some of the funniest I've ever read: his annoylogistic take on Neal Stephenson's Anathem, for example, or the epic assault on all 11 volumes of Robert Jordan's Wheel of Time, with extensive quotes from the master's prose. "He sounded like a bumblebee the size of a cat instead of a mastiff." Of course he did.
Someone said it's always better to read David Langford again than to read him banging on about criticism. [H'm. This pithy aphorism seems to need more work.]