Preferrably novel wise. But can easily try something else.

Others will probably be much more helpful than I will, but... Horus Heresy, maybe? I haven't read a lot of WH4K novels, but those should get you going. Also, if you're loaded, army books from the game are great and tend to help you get a good feel for the different groups represented in 40k. Plus, there is a lot of info on the web for free (wiki, Youtube, etc.).

novel wise is to start reading the books.
HH is good, but for 40 k you should read omnibusses, battle novels and "path of" books

In here: https://www.youtube.com/user/VaultsOfTerra

then rob a bank and read this books: http://www.blacklibrary.com/Home/horus-heresy-reading-order.html

Or, you always can go to the Wikia, which is based entirely on official material

http://warhammer40k.wikia.com/wiki/Warhammer_40k_Wiki

http://wh40k.lexicanum.com/wiki/Warhammer_40,000

Well, if you desire a specific single novel, i would suggest Strom of Iron by Graham McNeill. It sheds light on the Iron Warriors, gives Chaos and the Imperium a good showing and leads into his Ultramarines series. I would suggest saving the Horus Heresy until you've familiar with the core setting. That will allow its events to have far more impact.

I recommend Arch Warhammer channel ( https://www.youtube.com/channel/UChS9wazLlTUHSTeKVG39hZw ) , I found it very interesting, and said with great verve and voice.

Yea someone needs to tell him that the books are really expensive.

I said he should read them never said he needs to purchase them.

maybe local library, friends or something.
but yes damn expensive.

I asked Black library to make "monthly pass" to allow full access to their books for allocated time. to purchase them is just... 'spensive.

in the grimdarness of 41m there is only expensive stuff.