The primary mystery laid out in Proxima was solved in Ultima but it took a lot of effort to unveil who was behind the curtain. Much of the novel was set on a couple of alternate history Earths, with some trips to Proxima B thrown in.
After a really long and exhausting 500 pages, the book really peaks at the end. I thought it wrapped up very satisfyingly - a little glimmer of hope thrown in to a big pot of melancholia. Just the way I like it.