it is indeed still possible and the quests are very, very easy at level 100. it's also a fun and interesting chain so I'd recomend experiencing it even if you're not too bothered about changing the colour of your spells
They do drop, but the drop rate is low, and you are competing with lots of other people, both from your faction and the other. Not to mention that the loot is shared. I got death threats from another warlock when I autolooted the book from a shared kill. Its like a pilgrimage's to hell. But it is so worth it.
I would do a few circles of the Isle of Thunder first before buying it off AH. It was 20k on my server and took me 1.5-2 hours to farm it on Illidan-US (way over populated). Since I'm definitely not bringing in 10k/hour, it paid off for me. Never hurts to try!
The quest itself is definitely disappointingly easy at 100, but it's my own fault for not farming for it/trying at 90.