Janey Levy 
World of Microbes [PDF ebook] 

Microbes are everywhere. They’re on Earth’s surface, in the oceans, and underground, and they’re even on every one of us. While some of them can make us sick, like viruses, they’re also incredibly helpful; they generate at least half of the oxygen we breathe. Genetic studies of microbes have revealed much about life processes and the origin of life on Earth. Biologists take what they learn and apply it to other life forms. This captivating narrative describes the world of microbes, including prokaryotic cells, archaea such as extremophiles, and protists. A brief history of microbiology, and the characteristics of beneficial and harmful bacteria are also explored in this book.