All 3 clubs owned by same company and credits for one can be exchanged for another(if you have book credits you can swap for music or movie for example).
Please do yourself a favor(I get Nothing out of it) and check them out.
Movies= www.swapadvd.com
Music(all kinds by the way)=www.swapacd.com
Books(All kinds:Hardcover,paperback and even recorded books)=www.paperbackswap.com