To be very honest, I feel that most of the books out there on Metasploit are outdated, as it gets updated very often.
To learn more about the two, I would first download Metasploitable. This is an old configuration of Ubuntu with security holes for learning purposes.
The Metasploit and Backtrack FAQs are good places to start. Learn what each tool does, Backtrack consist of hundreds of tools, most of them you will probably never use. Take a look at your situation, and find the appropriate tool to use. Google is your good friend here.
Metasploit is simply an exploitation framework. The beauty of it is the ability to combine different exploits with different payloads to adapt to different situations. Metepreter is also an incredibly powerful shell to perform privilege escalation and other attacks from once you gained access.
Once you cracked Metasploitable, there are many other vulnerable VMs to exploit, herehere is a link to a question i had asked which has suggestions for many good practice boxes.
The key to understanding Backtrack and Metasploit is simply using it and getting familiar with the commands. Basic understanding of linux is of course needed, as many of the task in Backtrack are performed using the command line interface.
If you really want a book - this is highly recommended by others, although i have never actually read it.
I do not however, recommend installing the repositories on another OS unless it is really necessary. Why not use something that just works? Backtrack's theme is very cool to boot.