All manner of concrete, steel, and stone rubble from dredging, demolition projects, and other construction is used as artificial reef materials. This material is generally available at very low cost or free from construction companies who are more than happy to get rid of it. Transportation costs determine where this material is used by the Reef Program.
For those days that you can't get out diving, here are all the references listed in the sections of this site gathered together. Armchair diving beats watching the idiot box.
Nowadays, it is easy to order books from Amazon, Barnes & Noble, eBay, or in some cases, direct from the publisher. "eBooks" can be delivered instantly to your computer, phone, or tablet, and have become my preferred way of reading. Old out-of-print references can be found with an online search at any of the sites above and delivered to your door. All of these sites act as online clearinghouses for used-book sellers across the country, so it is easy to get almost anything.