It means "book".

If you hover your mouse over each of the icons at the top of the posting page (if you are on PC) you will see a small box with text in it, a one word description of the icon. If you are on a MAC not certain if that function works, though.

I have no idea why most people are using that specific icon, unless it's just because it's the first one in from (blushing). Obviously, not every post it is attached to is about a book.

BTW, I love the flexibility of this forum (although just a few moments ago when I went to colour my original text, when I did so, it blew away the text I had highlighted to colour. (Good thing I have a pretty good memory.)

Cheers!
Unis