I agree with gepon... the Sybex guide (by Todd Lammle) is the by far the best study guide for the CCNA exam (especially for subnetting!). You need to be careful because when purchasing from your favorite bookstore. The current exam number is 640-801. I just came back from Barns and Noble and was pricing some books I wanted to use to supplement my study program for the available commands in the CLI and I saw a Delux Edition CCNA study guide. I thought, "Cool... this may be exactly what I need." I was shot down in flames because the guide was for exam 640-607... it was outdated! To top it all off, the bookstore still wanted to charge full price anyway!
If you are interested, two books I would recommed to get a good grasp on the Cisco IOS are the CCNA Portable Command Guide by Scott Empson (Cisco Press) and Cisco IOS In A Nutshell by James Boney (O'Reilly & Associates). These explain some topic in greater detail that the Sybex guide briefly cover (especially NAT).
I agree, Sybex gives another view of the same topics but from a slightly different angle and is also fun to read.
One book which doesn't get mentioned much is
Computer Networks 4th Edition by Andrew Tanenbaum, March 2003
I use this book as well as Sybex and Cisco Press as it goes into greater detail about networking concepts and theories involved in CCNA. (Remember CCNA is the building block most of the other exams are built upon) Almost no Cisco stuff in it though.
The 2nd edition of this text was used as my course book at Uni for the Networks 1st year course. The 4th edition is fairly upto date and proves to be a great accompanyment to the Sybex and Cisco press.
Most importantly though, it doesn't matter how much stuff you have. The hard part is reading it all and remembering it.