This is a serious answer: Hutton's Arse, by Malcolm Rider. A magnificent book on the topic of three billion years' geological history of the north-west Highlands of Scotland. My son bought it for my birthday and I read it through three times non-stop. If you know the area you really can't not have this book!

I also have a penchant for reading use-of-English books again and again, and I have a wildflowers book by Marjorie Blamey that is my favourite bedside dipper.