the legal use for them is growing unusual fruit & vegetables, though some do use them for other purposes. It's also not illegal to own or sell books like this -
Grow Great Marijuana: An Uncomplicated Guide to Growing the World's Finest Cannabis by Logan Edwards
Even amazon sell it -

in the UK
http://www.amazon.co.uk/exec/obidos/ASIN/0977650502/?tag=yah.007-21

and the USA
http://www.amazon.com/exec/obidos/ASIN/0977650502/?tag=yah.007-20

Though you can get into trouble if caught using them for growing weed!