There's a Vendor glitch that allows you to buy multiple versions of a skill book. When you buy the book, it will disappear from their inventory, but once you use it it will reappear.

Simply sell whatever skill book you wish to duplicate to a vendor, then close the dialog and buy the book back. Talk to the vendor again after using the book, and they'll now have another copy available for sale.

After bringing up the console type in "cmdlist" and click submit.
Give it a sec and it will bring up a LONG list of codes you can use.

Begin Game With Console one and press ~ while playing to activate the cheat menu then type "god" to unlock God Mode.