I've been reading a lot of RPG Books for a while.. even though I have not played any. I just love the CoC aesthetic, I sorta read them as books... my hope is to one day run these games....
BUT.... the biggest one I have personally read is this...
Masks of Nyarlathotep ... it is a beyond epic. The book has 3 entire books in it to make up the quest. It has these awesome sub-scenes.. where you take on characters in a different time zone and live out the events affecting you. So like instead of being told about something that happened, you can choose to actually go and do it yourself. The Slipcase comes with 3 books, hand outs, maps a screen.. to play this out fully is liturally years of your life.
Horror on the Orient Express is the other massive campaign I know of. It is not as big as MoN. Like MoN though, it is a reprint of an old CoC thing for the current edition with a ton of new stuff. It is similar in a scope but this time it it is only 2 massve HC books. I have not played these games myself, only read them and imo they seem like they both are ubr good.