The Salmon of Doubt
This posthumous cash in contains random articles, essays, speeches and letters written by Adams before he died as well as a hitchhiker's based short story and the chunk of a Dirk Gently book that never was that gives this volume a title.
No single part of this book is worth buying it for, but as a whole it is a nice reminder of who Douglas Adams was, what he stood for and the work he was capable of.
I bought this book not long after it made paperback for less than half price in a supermarket. I don't know I would have bothered if it wasn't so cheap. That I only just got round to reading it speaks volumes as far as I'm concerned.
My recommendation, unless you're a Hitchhiker's nut, is that you don't buy this for less than a bargain price.
Diamond Dogs, Turquoise Days
This book is made up of the novellas that comprise the title. Both take place within the Revelation Space universe and shed light on various aspects of Reynolds' creation that aren't discussed elsewhere.
Diamond Dogs is about a group of adventurers who attempt to solve the puzzle of an alien artifact known as the Blood Spire. Nothing new here, in fact many of it's ideas are cribs from other better realized works.
Turquoise Days is the more interesting of the two pieces, but still nothing remarkable. This one is set on an isolationist Pattern Juggler planet and follows a researcher called Naqi as she deals with the legacy of the Jugglers and the implications of the arrival of off planet visitors. It's too short to deliver on it's promise.
I'd only recommend this to someone who has become hooked on Reynolds' universe and wants to know more. Outside of that context these are pretty forgettable little pieces