Well, personally, I would say the 2 key and primary books to study for your CCNA are by:

1) Wendell Odom

2) Todd Lammle

I alternate between each book. So I'll read a chapter from the Odom book, and then read a chapter from the Lammle book.

Lately some people have been heavily criticizing Lammle for a few mistakes in his book. But I don't find those errors to be a big deal, since I'm also reading Odom's book at the same time.

I find it REALLY helps to use 2 core books, from 2 different authors who explain things using different styles. So I would NOT recommend just depending upon the official Odom book. Instead, the Lammle book is still very important to me, and has helped me a lot (despite the errors).

In addition, I'm also using:

3) Computer Networking (A top down approach) by: Kurose & Ross.

This is a book that is often used as a university-level introduction to computer networking. I really like it so far.

4) Cisco CCNA in 60 Days (Paul Browning).

5) CCNA Nuggets Videos, by Jeremy Cioara

6) CCNA Videos by Kevin Wallace

7) CCNA Todd Lammle Videos

8) Network Warrior (book by Gary Donahue)

9) Comptia Network + (book by Kevin Wallace and Keith Barker)

10) TCP/IP (book by Charles M. Kozierok)

NOTE: if you don't have time for all that, then you should just stick with:

A) Odom book

B) Lammle book

C) Nuggets videos

And FINALLY... if you don't even have time to use all 3 of these different resources, and had to just pick one from the last above, then of course you will want to stick with the Odom book only.