We're all ­readers here, right?

I mean, you probably wouldn't be ­hanging around the book pages of the Sunday Herald if you weren't. But here's the ­question - are you a re-reader?

Jo Walton, as the subtitle of What Makes This Book So Great suggests - Re-reading the Classics of Science Fiction & Fantasy - definitely is.

A science fiction author, her collection of short essays is concerned with how our attitudes to our favourite books can change and evolve over the years. Sometimes that second encounter (or third or fourth) will be something of a disappointment. As she says, the "temporary shininess" that attracted you first time around may have worn off, or on second reading the book appears shallower than you recall.

Sometimes, though, re-reading offers a greater pleasure: "Because I know what's coming, because I'm familiar with the characters and the world of the story, I have more time to pay attention to them. I can immerse myself in details and connections I rushed past the first time and delight in how they are put together. I can relax into the book. I can trust it completely."

I'm not sure you can say the same about this book. It is not Walton's fault, really, but this is essentially a collection of her blogs. And blogs tend to be starting points, not the final word. Half the fun of them, after all, is the conversations that go on below the line.

That couldn't be any more evident here on those occasions when a column ends in a question. You want to know what answers she received. You could look them up, of course, but if you're reading a book that option isn't just a mouse click away.

There are other problems. These blog entries are short - you might even say slight. That's almost inevitable in their status as conversation starters. More than that, collecting them in book form also endows an extra significance on the words and as a result what appears chatty and accessible online can feel ­unconsidered and cliched on the page. ("They don't make them like that any more" is one sign-off.)

So it's not a great book. But on this evidence, it is an interesting blog. Walton explicitly says she's a reader not a critic. But her reading is intelligent, informed and engaged. There are lots of fascinating ideas thrown up and out here. About reading, about re-reading, about science fiction and about literature in general, delivered.

She's sparky and provocative (and funny), firing off sentences that will intrigue and irritate both mainstream and science fiction readers. "Why is it," she writes "that Iain Banks writes these mainstream books with great characters and voice and a strong sense of place and then writes SF with nifty backgrounds and ideas but almost lacking in characters?"

And isn't it a pity, she says, that George Eliot didn't write SF, "because she saw the world in an essentially science-fictional way. She saw how technology changes society - she understood that thoroughly." Eliot's world view, she argues, is much closer to Wells than Dickens.

Most of her reading, though, is in the deep stream of genre. I am, I admit, a casual SF reader these days and apart from the foundation names - Asimov, Heinlein, Arthur C, Le Guin, Samuel R Delaney - and mainstream writers edging towards SF and fantasy - Nevil Shute, Salman Rushdie, Alan Garner - many of the writers she explores are new to me.

That's not necessarily a bad thing. I've added Raphael Carter's The Fortunate Fall and Karl Schroeder's Lady Of Mazes to the huge list of books I mean to get around to reading one day. And like her, I think Jack Womack's Random Acts Of Senseless Violence is a criminally ignored book; if her encomium on this novel encourages anyone to seek it out her book is worthwhile.

Ultimately, though, what's most interesting about this collection is the picture of Walton the reader that emerges. Inevitably Walton's interests and predilections come to the fore through the choice of books she writes about.

And so, above and beyond genre, we find a writer engaged in issues of technology, feminism and gender as well as the poetry of words and ideas. In the end, this blog-turned-book is really a form of biography. As such, it's a reminder that we are what we read. Or what we re-read, as the case may be.