Hitchhiker's, volume 5.
There are some good lines in this, but I can't help feeling it would have been better if Adams had left it unwritten, or at least unpublished. It is very disjointed, with Ford, Arthur and Trillian mostly in separate stories.
It starts in what would be a parallel universe - if such things existed, which they don't, because "it makes as much sense as the sea being parallel".
"If there was one thing life had taught her it was that there are times when you do not go back for your bag and other times when you do. It had yet to teach her to distinguish between the two types of occasion".
"The messages that one part of her brain was busy sending to another were not necessarily arriving on time or the right way up".
"For something she hadn't expected... it wasn't going the way she expected".
Maximegalon Institute of Slowly and Pointlessly Working out the Surprisingly Obvious.
The future is "just the same old stuff in faster cars and smellier air".
"It occupied the same co-ordinates in space time [as Earth:]. What co-ordinates it occupied in probability was anyone's guess".
"The sun was quite bright but the day was hazy and vague".
"A common mistake... when trying to design something completely foolproof is to underestimate the ingenuity of complete fools".
"Her mood swings were very unpredictable but so far they'd all been between different types of bad ones... She had been sent as a test of his faith, if not his patience".
AmEx "gave cards exclusively to just about anybody".
"about three other customers... it was not the kind of place that you felt like being that specific in".
"The possible continually interfered with the probable".
Brief summary and favourite quotes from the other four of the five books, as follows:
Hitchhiker's Guide (vol 1): http://www.goodreads.com/review/show/107767859
Restaurant at the End of Universe (vol 2): http://www.goodreads.com/review/show/107768059
Life, the Universe and Everything (vol 3):
So Long, and Thanks for all the Fish (vol 4): http://www.goodreads.com/review/show/107768203