Also done in an excellent though unfinished series of books, The Salvation War[0]. Albeit travel was only a side effect of the real goal there.
[0] - http://www.tboverse.us/HPCAFORUM/phpBB3/viewforum.php?f=29 - full text of Armageddon and Pantheocide available there; the author decided to abandon the series because someone stole his work to destroy his ability to publish it.
There are some details scattered around the forum. For starters, one quote from the author[0]:
"The third part never got written. We had a contract signed to publish them and they'd been prepared, copy-edited and put into paperback novel format (I actually have the author's preprints) when a . . . . person . . . . stole the copy and published it himself as a torrent. As a result, the whole deal fell through and nobody will touch an already-published work. So, without any possibility of generating return, I ditched the project. There's no chance of going back to it now."
I once saw a more detailed story about the guy who stole the copy and alleged reasons for doing it, but I can't find it right now.
[0] - http://www.tboverse.us/HPCAFORUM/phpBB3/viewforum.php?f=29 - full text of Armageddon and Pantheocide available there; the author decided to abandon the series because someone stole his work to destroy his ability to publish it.