I'll have to mention Monte's 2e 'The Astral Plane' supplement for Planescape as the best source on the Astral out there.
4e's 'The Plane Above' was mentioned as well above, but it isn't exactly a true sourcebook on the Astral plane (or the Astral Sea in 4e terms) though as I see it. It does cover the 4e version of the Astral, but it also covers a lot more pages looking at divine domains and so-called shattered dominions. A lot of the content isn't on the Astral itself, but what would have been entire outer planes, divine domains therein, or demiplanes in prior editions. My own just plain not liking the 4e cosmology aside, I think the book suffers from trying to do too much, and rather than being focused on the Astral itself, it tries to stuff the entirety of what would have been the outer planes sans the Abyss, and monsters therein, in one source when it should have done multiple sourcebooks IMO.
Completely different game and cosmology, but nWoD Mage had some interesting material on its own Astral which is IMO worth mining for ideas.
Pulling from real world material, looking at some of the early 20th century Theosophy material (Blavatsky et al) has its own various conceptions of 'the astral' worth consideration.