Nutshell: earthlings begin building skyhook, aliens show up, aliens go away, earthlings finish skyhook, yay!
Nifty parallel drawn between ancient monument builders and scifi megastructures through the use of an ancient Sri Lankan legend (or what purports to be, anyway). Lotsa technical detail. Whatever. Best parts of the book are the political interactions between interest groups regarding obstacles to building the space elevator. The main one, set up as structural to the narrative early on--religious opposition from monks on the only practical site--is removed by clever but literal deus ex machina (164).
Most engaging sections involve the alien probe, which effectively ridicules earthling stupidity, especially religion (92-95). I'm just not sure why the aliens are in the book, though.
Ending of the novel more or less ruined by a thriller-rescue sequence. Definitely a source text for [b:Red Mars|77507|Red Mars (Mars Trilogy, #1)|Kim Stanley Robinson|http://d.gr-assets.com/books/1320484020s/77507.jpg|40712]. Cool that the novel quotes a line from Goethe that would be eponymous for Asimov.
Recommended for those paying substantial premiums insuring against every possible future, persons somewhat tired of establishing precedents in interplanetary law, and top-hatted Victorian personages.