I'd recommend Against the Day, then if you enjoy that, try Gravity's Rainbow again. It continues some of the timeline of Against the Day. GR is super tough for the first third but gets easier and more enjoyable.
I don't remember AtD as being particularly filthy, but it has been a long time. It remains one of my all-time favorites. If nothing else, I'm indebted to the book for encouraging me to seek out literary westerns such as Warlock by Oakley Hall and Butcher's Crossing by John Williams.