Actually, a good place to get a good basic understanding on a Cisco site:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/
This is the online version of Cisco's Internetworking book. It covers most topis at a basic level, but still gives you a good idea of how all the various technologies and protocols relate to each other.
Beyond that, most / any of the CCNA study books would be good. Which one works best for you is going to depend on you ... of the spectrum of available books, you need to find one that fits your preference.
Many people find the Todd Lammie books very good, some don't like his style of presentation. It's an individual thing.
Go to a book store when you have some spare time, read a similar chapter from several CCNA books, and decide which one works best for you.
Good Luck
Scott
