Why a book about two completely different subjects under a single title? SAN is "Storage Area Network" - think "attached to the computer" like RAID. NAS is "Network Attached Storage" - think "Network File System". Personally, I wanted to just read the NAS information. However, to answer the question, SAN and NAS are converging technologies. They aren't there yet, but they will be.
Larger companies are starting to suffer under larger amounts of data. It's no longer enough to max out the disk slots on your tower server, even with today's highest density disk drives. Administrators are turning to these technologies to help distribute and manage the hardware and data.
These technologies are mostly a game of dealing with vendors, and the author lets you know about both the technology side and the vendor side. There are explanations of various issues that you will have to contend with, such as choosing various solutions for each technology.
Did you think these technologies would let you avoid doing backups? Think again! While you may gain some data integrity and security with SAN/NAS, they are still no substitute for a good backup strategy. But the author does not leave you in the dark. Rather, he makes a point of dealing with backing up huge data stores, and shows you the new possibilities that these technologies offer.
Are you ready to move up? Grab this book (and your chequebook - these aren't cheap!) and get moving…