www.thatfishplace.com (www.thatpetplace.com takes you to the same site) is a great mail-order website that carries tons of aquariums books, magazines, and even posters. Click on the "Aquarium" link near the top, then "Books, Media & Posters" from the menu that appears on the left. Hopefully, you'll find something that will appeal to them! Good luck!

There are a lot of great books on marine aquariums. If they are advanced marine aquarists then consider any of the three volumes from Julian Sprung and J. Charles Delbeek (these are pricey but they are the ultimate):

The Reef Aquarium Vol.1 A Comprehensive Guide to the Identification and Care of Tropical Marine Invertebrates

The Reef Aquarium: Vol 2 A Comprehensive Guide to the Identification and Care of Tropical Marine Invertebrates

The Reef Aquarium: Vol. 3 Science, Art, and Technology

Another great book is Aquarium Corals by Eric H. Borneman (Excellent book and less expensive than the previous.three.)

If they are less experienced you might consider one of these:

Natural Reef Aquarium by John H. Tullock
Conscientious Marine Aquarist by Robert Fenner

You can get these at Amazon.com or marinedepot.com