About an hour or two ago, Marc Laidlaw (ex-Valve-and-Half-Life writer) posted a letter, which is quite obviously the plot outline for Half Life 3. Seems designed to give people some closure, in lieu of the fact that HL3 is never coming out.
His website has been hugged to death, copies are here:
Apparently, the NDA for HL3 expired recently, which may explain the timing.
It looks like the book is closed people, the saga is over.