Read More - Read Better

I was looking at onlinebookclub forum the other day, which is quite a good book discussion site, and someone was asking for advice. Aspiring writers are often told to "read more", and this particular aspiring writer wanted to know what sort of books they should read to be a better writer.
I gave some advice that had been useful to me, and I thought I'd like to expand on it here.
The first thing to do is to choose a book that you have already read, but which you didn't think was all that good. (Don't try to do this with a book you really, really like - you'll never be able to read it in the same way again!). It doesn't matter what the book is about, or who it's by - just choose a book that you finished, and then thought; "Well, that was average."
The next thing to do is to start re-reading it, and this time pick it apart. Did this particular scene confuse you? Why? How could it have been written better? What would you have included or left out to improve it?
And what about that character - the one who suddenly does something that makes no sense. Why did they do that? How would you have written it to keep the character consistent?
Or that description? Did you feel you were really there, in that place? If not, how could it have been re-written to improve it?

A good example of this for me (though this is a book I love, rather than one that I consider to be merely average!) came when I re-read Warrior Scarlet by Rosemary Sutcliff as an adult.
When I was in my first year at grammar school (so eleven or twelve years old), we studied Warrior Scarlet in English lessons. In those days, there was no national curriculum, so teachers were more free to choose books to read as a class than they are now. That year, we read Little Grey Men, by BB, Edward Lear's poetry - especially The Dong with the Luminous Nose and The Jumblies, and Warrior Scarlet. Nobody thought about the fact that an all girls' school was studying texts which hardly included any female characters (there are female characters in Warrior Scarlet, and one girl has an important sub-plot, but in the other texts we studied that year, female characters were invisible) - but it was the 1970s and things were different then.
Warrior Scarlet is about a Bronze Age boy becoming an adult of his tribe - and for the boys of the tribe, this means hunting and killing a wolf. Failure to do this means that the boy cannot be accepted as an adult of the tribe, and becomes part of the "servant class" of the Small Dark People who live on the fringes of the tribe. And they only get one try at it.
So an important scene in the book is the wolf hunt.
I remembered this scene in great detail from my first reading of the book - I was there, in my imagination, creeping through the undergrowth with my bronze tipped spear, examining wolf footprints and looking for wolf hairs caught on bushes. It was all very vivid.
When I came to the scene as an adult, none of that was there! It had all come from my own imagination, with only a few sparse descriptions from the text itself. There's the mark of a master storyteller - telling just enough for the reader's imagination to take flight and fill in the gaps for themselves!