I've been reading SF for more than 30 years.
I've probably read everything worth reading in the field, and I’ve been always intrigued by the two questions:
1 - What makes a SF book a good example of its kind?
2 - Why is SF relished by practiced readers, while others hate it?
Walton's book tries to answer the above-mentioned questions. Walton is clearly a SF devotee (on top of being a SF writer as well, which I've never read in fiction mode by the way).
One crucial factor is that SF is written in a kind of code, which must be learned by apprenticeship. This necessity, of course, intensifies the skeptic's bewilderment at the bother taken by those who learn it in the first place. I learned all of the SF narrative codes when I was very young.
SF reading is like learning a language, ie, the younger we are, the better we'll be prepared to fully appreciate it. If one tries to start reading SF later in life, it's almost always a hit or miss situation. Most of the time, it's a miss. For those readers for which it’s a miss, it usually means they lack proper “tools” to able to fully engage with the text.
In this book, Walton does not try to attempt a deep and rigorous approach to SF criticism (vide her last essay in the book “Literary criticism vs talking about books”; two other essays worth reading are “Why I re-read” and “Do you skim”). What she aims to accomplish is a way to give us a particular take on some very distinct works in the SF field. Most of the time I don't agree with her (eg, her analysis of the Vorkosigan saga by Lois McMaster Bujold, which Walton elevates to Nirvana status, but that I particularly hate; that's for another occasion; on the other hand, her deep reading is spot on in some cases: eg, “Icehenge” by Kim Stanley Robinson: “He reads Cavafy, but he breathes T. S. Eliot”, making an intertextual comparison between both works).
Speaking for myself, when I embark on a SF text, what I look for is estrangement, ie, the ability to search that ethereal quality of transcendence. It goes without saying that the ability to provoke transcendence is not exclusive to SF. The function of estrangement in SF is not identical with the kinds found in certain other narrative texts (eg, mainstream/mimetic literature). For example, we can value History for its ability to give us estrangement of other ancient times and what they can tell us about the present, ie, History is interesting for its alterity. Unfortunately History, no matter how imaginatively depicted, cannot go beyond its own limits. Only SF can/may enter into the truly other (vide novel "Stars in My Pocket Like Grains of Sand" by Samuel Delany, which Walton also “analyses”, for a wonderful example of this; it's also one of her best essays in the book).
What seemed really strange in this book was the absence of negative reviews/opinions. For someone as well-read as Walton, it’s weird. Only a handful of essays contain mild and en passant “negative” opinions (eg, “The Mars trilogy” by Kim Stanley Robinson, James Tiptree Jr’s biography by Julie Phillips).
Bottom-line 1: I read SF for the meta-reading it allows me. It’s an endless journey…
Bottom-Line 2: After finishing it, I felt like (re-)reading some of the referenced novels (eg, “Tam Lin” by Pamela Dean, which I’ve never read; “Neuromancer” by William Gibson, which I’ve read multiple times, “Anathem” by Neal Stephenson, ditto, “Biting the Sun” by Tanith Lee, “Stars in My Pocket Like Grains of Sand” by Samuel Delany, “China Mountain Zhang” by Maureen F. McHugh, which I’ve read late last year, and in my opinion it’s one of the best (SF or not) novels ever written – vide review here).
SF = Speculative Fiction.
Off-topic: I was thinking of doing a particular post about this, but I won’t bother. Here it goes. I’ve been receiving emails to clarify what I mean by “speculative fiction”; this was first used by Heinlein, and it’s a broader term that comprises everything an author is able to create, ie, where anything can happen. It’s a place beyond reality, a place that could have been, or might have been, if only the rules of the universe were altered. “Speculative fiction” goes beyond the mundane and takes me into a world of might-have-been, science, intertextuality, transcendence, otherness, estrangement, etc. It’s a world where I leave part of myself behind when I return to the universe as I know it. It pushes the limits of the imagination. A good SF story makes me think, and also provides a new insight into human nature or even give me a new outlook on life. It’s a tall order I know, but that’s what I always aim for in a good SF book. Even if after reading this, you still have doubts about what SF is (or isn’t), I’ll give you a clue: It’s not about magic swords, spaceships, robots, dwarfs, leprechauns, pixies, elves, and the like. SF allow us to see the world with new eyes. SF is about you and me.