Ultimate Exposure: All You Need to Know to Take Perfect Photos with any Camera by David Nightingale
Exposure is photography's fundamental concept. Master it, and you'll master photography.
You don't need a book to take good photos any more; most (but not all) of the time your camera, or your phone, will handle things for you. But if you don't know what it's doing - you don't know how to expose photos - you can't join in the real fun of photography.
With this book, though, you'll quickly master the key principles, and grasp a few simple concepts that will open up a world of beautiful sunsets, clever silhouettes, light trails and bokeh - as well as sophisticated techniques like HDR, manual exposure blending, and focus stacking.
Armed with this knowledge the digital photos you share will stand head-and-shoulders above the rest, and you'll have no problem using that film camera or used Polaroid you picked up at that garage-sale.