The responses there should answer your questions. (Most LFLs have a sign saying "Take a book, leave a book," which basically sums up the "book-swap shelf" concept, but apparently people do get confused; at one of the new LFLs in my town, somebody was dropping off actual library books there, as if it were a book-return!)
On another note; any advice on the best place to release a book in the wild? Indoors or outdoors; city or a more rural area. Thanks
You can release a book any place you want as long as you think its safe. With this I think of security area's where people might think its a bomb. When out doors you might want to put it in a clear plastic bag to protect it from the weather. Just use your imagination and common sense. Good Luck.
I noticed free little libraries that were on the map but aren’t listed on BookCrossing. Should I contact the owner of that free little library?
No, I would not do that. Not all Little Free Libraries need to be listed on BookCrossing. BookCrossing Zones are places where a member of BookCrossing has released one or more books; if no one has ever released a BC book in a specific LFL, it makes sense that it's not listed on BC.com.
Likewise, not all Official BookCrossing Zones are LFLs, either. I have a public bookcase out in my front yard that is an OBCZ but it's not listed on the LFL website. That is fine too.