Good lattice theory books?

A good, user-friendly, modern, introductory textbook is Davey and Priestley's Introduction to Lattices and Order.

Incidentally, Gian-Carlo Rota used to say much the same thing as you, Zev: that lattice theory had been robbed of its rightful place in mathematics.

I've used Garret Birkhoff book "Lattice theory". Could be a bit outdated nowadays, but it gives a deep feeling. Not so sure if it is good as lattice field is not my field.

George Grätzer has written a couple of well-regarded books on lattices. The Wikipedia page recommends his "Lattice theory. First concepts and distributive lattices" and several others.