What’s the most useful book you’ve ever read? And, why?
Most useful? Hmm... I've been reading for about 50 years by now, and different kinds of books have been useful in very different ways...
*The first that comes to mind is the Bible. I must say I have used that a lot over the years. There is no other book I have in so many translations; and it has been helpful from many points of view: in understanding myself, other people, God (hopefully), religious beliefs, history, culture, and references in other books by other authors... On the other hand: Other books and people have also influenced my interpretation of the Bible! The most important "other author" that way for me is probably C.S. Lewis - his fiction as well as theology.
*There are a number of Swedish authors whose influence have been important or "useful" to me when it comes to understanding the history of my own country. One series of (four) books that may be worth mentioning in international context is The Emigrants by Vilhelm Moberg (written in the 1950s), about a Swedish family's migration to Minnesota in the mid 19th century. These were among the first "adult" novels that I read in my early teens.
*A series of books that proved "useful" to me in the 21st century is the Harry Potter books. First, listening to them as audio books helped me through some bad times when I had to spend a lot of time just lying on my back. Later, discussing them with other people on an internet forum also inspired me to read and reread and discuss a number of old classics, and to get into the habit of googling and reading up on all kinds of odds and ends referred to. You could say these books kept me entertained and inspired at a time when I really needed that kind of distraction and brain exercise. It also played its part in later getting me to move on to blogging. (If anyone is interested - some of my reflections on the HP books I have tried to collect in a separate blog, Through My Spectrespecs.)
Other kinds of books I've found useful, even if I can't claim to have read every page in them:
*Dictionaries (lots of them).
*My old school atlas from 1964, even though political borders and names have changed quite a bit since then. I do have updated ones - and now of course there is also the internet. But the old one still represents how the world was first presented to me. Sometimes I return to check something.
*The first cookbook I got (probably from my mother?) when once upon a time I moved away from home. I have later ones, but for some things I still return to the old one.
*Lately I've made quite frequent use of my Flora to identify wild flowers and trees. This came with increased interest in photography, and blogging!