So Long is the best in the "trilogy of four" of the Hitchiker's Guide to the Galaxy, after the unexpected surprise of the first, and the two funny yet low-paced sequels. There, Adams finds out a solid thread to elaborate upon, even if a classic one and at the detriment of his zany humor (there's still the rain god, though).
Arthur gets back on earth where he falls in love with that lady who had a glimpse of the ultimate truth, of the meaning of it all, but just when the earth evaporated under a giant laser beam to make way for an hyperspace bypass. Now she forgot it, and they manage to find it back together.
The love story is touching and incredibly realistic, while of course still and always narrated in this weird, delightful, illogical---or may be too logical for literature---funny D.G.'s free wild style. But most of all, there is a real overall meaning. Whereas 42 means nothing, God's last message to His creation bears a genuine message of tolerance and encouragement to keep satisfied with life and all that comes with it. The allegory of the otter pulling the raft is deep and couldn't explain it best. The laughters of Prax about Arthur illustrates simply how ludicrous can be the metaphysical wonders. This last book is full of metaphors like these.
I'd like to point out also how close to Monty Python's Meaning of Life it seems to me, with a development of the whole story before the "secret" much the same, full of idiocies and funny details of life. That shows another connection in this regards (and if we are to believe Yoakum, even 42 stems from Pythons, somehow).
There was a point to this review, but it has temporarily escaped the reviewer's mind.