Slices and dices through clay, roots, plastic, burlap, and twine while never rusting or pitting. You won't need another gardening tool, even if you're in the Swiss Army. I prefer this A.M. Leonard version but you may like others, just make sure you get one with a brightly-colored handle.
Let's quit sprayers with handles that break off when cut on a branch or stem, or get brittle in sun and cold; I've got a bucket of them in my garage I can't stand to throw away. Fireman style nozzles are where it's at, and this Bon-Aire offering has remained functioning and durable as I sling it on the ground with reckless abandon.
Dutch Hoe or Push Pull Hoe
Choosing this type of tool is more about how you work and what you prefer, just forget the traditional hoe; there are many brands and styles so you might have to experiment. Use this tool in a gliding motion to easily pull up weeds from beds with looser soil (think veg, loam, or sand) without bending over or stabbing the ground.
Climate-Wise Landscaping: Practical Actions for a Sustainable Future -- Sue Reed and Ginny Stibolt
Learn how to shrink your carbon footprint, clean air and water, provide habitat, cool your home, and much more. Lots of inspiring quotes throughout by noted authorities in garden design and environmental thinking to punctuate this easy how-to guide.
Geared toward intermediate and advanced gardeners, this book will show you why and how to think of plants not as objets d'art but as parts of an ecological community that evolves over time.
Principles of Ecological Landscape Design -- Travis Beck
A slew of ecological principles that can be translated to garden design from small to large scale. It's a more technical book but written with grace and accessibility, as are all of the above books listed here.