This book is darned near perfect: It's about how folks navigated back before there was a GPS phone in every pocket. Had things I didn't know about Vikings and about Marshall Island stick chart wave maps. There's an interesting theory that Viking "sunstones" might have been chunks of calcite "Icelandic spar", a naturally-occuring polarized crystal that could help you spot the sun's true direction on an overcast day. Celestial navigation is different if you're in the tropics because the stars behave more consistently. How to estimate the size or distance of a far-away thing by seeing how many fingers on an outstretched hand are required to obscure it. "Starboard" got its name from a steering-board that hung off the right-hand side of Viking cargo vessels. This book is full of neat stuff like that. Plus there's an excellent bibliography so I can learn more about what we've rediscovered about Marshall Island stick chart wave maps and wave navigation in the last couple of decades. (The stick charts aren't so important for wave navigation, it turns out, but they sure are intriguing artifacts to stumble upon in museums.)