There are a lot of programs to use for retrieving mail. The first one that springs to my mind is Thunderbird. Can you go into a little more detail of the approach you intend?
If you want a book about the shell and the programs you can maybe start with The Linux Command Line
. It's available as a free-of-charge e-book or can be purchased from your local book store. In the beginning it might be easier to buy the paper edition and use it along side your computer. If nothing else, then you are voting with your wallet that you want more such books.
About all the other "commands" on your computer, they are just programs that you have on your system, either pre-installed by your distro or added after installation by you yourself. For example, even "ls" is just a program on the system. You can string several programs together with a pipe to get an inventory of what is on your system.
apropos -w '*' | awk '$2 == "(1)" || $2== "(8)"' | sort -u | more
Then interesting ones can be explored with "man"