In the travel guides (Hawaii: Big Island Revealed, etc.), they give codes like "hh20" or something like that to get to their lodging reviews online for some islands, with a different code for each book (newer editions.) My question is whether you share these codes with your friends so they don't have to buy the books? My sister and I were debating whether you should........

Absolutely immoral practice.
Instead send them to their local Barnes and Noble to look it up on their own.

I would give the codes to a friend sitting in my house or on the phone with me, but I wouldn't post them here on this site. Of course, you can go in any bookstore that carries them, and copy them right out of the "Revealed" guides.

Public libraries typically carry travel books that circulate for several weeks.

I haven't seen the Doughty-Friedman "Revealed" series in our library here. Don't know if they would get those........... So far as I know, they are the only guides that put some of their lodging reviews online (instead of in the book), then give you a code to get into the site. But, yes, we have several other travel guides in our library.