LaTeX Books online
I was looking something up today, and quite by accident found that Safari Books Online has both the Guide to LaTeX and The LaTeX Companion available. Unfortunately, the subscription costs mean that for individuals the price is far too high, compared to buying the printed versions. However, I also found that my employers (UEA) have paid for access to a range of books via the service. So if you are in a university it might be worth seeing if you can get the books as part of your institution’s more general access policies.