Books: Where to start?

I've not played 40K since the 90s. I have vague memories of some major events like there being a Horus Heresy and some knowledge of Space Marine chapters (Blood Angels are mad, Ultramarines are dull, etc) and Chaos Gods (Nurgle is disease, Korn is angry, Tzeentch is knowledge and scorcery).

I want to start reading on a good Space Marine book but there's hundreds!! I thought about just picking the best rated on Good Reads but you seem to require a lot of background for some of them so I'm hesitant. E.g. Ragnar Blackmane has been recommended but I don't know enough about Space Wolves for the blurb on the back to make sense. Cadia? Flesh Tearers? Dark Angels - I only know they had cool dark-green armour in the 90s.

So any advice on where to begin that would be a an easy intro would be much appreciated!

Likes:

• Space Wolves! Who doesn't like viking-space-wolfs? I'd love to learn more.
• Magnus the Red (also a good guy and tragic - I only know this as I spent hours reading wikis!). Shitty armour though!
• Chaos (esp. Tzeentch who I'm convinced is a true god and the only "Good Guy" in 40K)
• The Emperor (evil bastard but interesting)