The War of the Ring: The History of The Lord of the Rings, Part Three by J.R.R. Tolkien and Christopher Tolkien

This covers the second half of Two Towers and the first half of Return. Less fumbling about for inspiration than the opening -- but some, with the future Orthanc stone first being just shattered when it hits the ground, or Faramir's name and relationship to Boromir -- and a lot of work with time tables. (This is what happens when you split up the party and four separate courses of events have to interweave.)