Reddit mentions of Zhuangzi and Early Chinese Philosophy: Vagueness, Transformation and Paradox (Ashgate World Philosophies Series)

If you're interested in learning more about the history of the religion, I think that the books Lukeweiss has recommended will serve you well. If you're interested in studying some of the primary sources in greater detail, to understand the key early texts of Daoism (namely, the Daodejing and the Zhuangzi), then here are a few recommendations for you. Of course, you might choose to read these two books before studying them in greater detail, in which case, look here or here for the Daodejing, and here or here for the Zhuangzi. Now then:

Allan, Sarah. The Way of Water and Sprouts of Virtue.

Coutinho, Steve. Zhuangzi and Early Chinese Philosophy: Vagueness, Transformation and Paradox.

Religious and Philosophical Aspects of the Laozi. Mark Csikszentmihalyi and Philip Ivanhoe.

Schwartz, Benjamin. The World of Thought in Ancient China.

Slingerland, Edward. Effortless Action.

Those are the first few books that are coming to mind - they'll certainly help you get started on understanding these important Daoist texts. I think that the Slingerland book and Benjamin Schwartz's book might serve you better as a general introduction to important ideas in Daoist texts, and then the other books can help you delve even deeper. Although, if you are more interested in learning about the history of Daoism than the careful study of early Daoist philosophy, I definitely have to second Lukeweiss' suggestion of Introducing Daoism.