I just discovered that there is another book about TeX that has been “freed”:
TeX for the Impatient
ftp://ftp.tug.org/tex/impatient-1.0/book.pdf
I have not found time to read it yet, but flipping through the PDF suggests that this is a pretty good book.
As for why you would want to read about TeX? Well, at least you can tell confidently which one of the following five gives a different (probably unintended) output:
1. H{\aa}stad
2. H\aastad
3. H\aa
stad
4. H\aa{}stad
5. H\aa stad
and perhaps even explain the (visually non-existent) differences between the other four. 
P.S. I feel obligated to make sure you really know 2 is wrong unless you really have defined \aastad. 1 and 4 are better because they are less confusing. A control word “eats” the spaces and also the end-of-line after it, hence 3 and 5 both “work” but for a less obvious reason.
