My love of Taoist philosophy lead me to add a BA in Eastern Philosophy onto my Computer Science degree (One I like, and one to pay the bills. I'll let you decide which is which.) While writing my senior research project, I discovered Welch's Taoism: The Parting of the Way. I just reread this book after a lifetime of reading writings, translations and interpretations of Eastern Philosophies. Welsh does an admirable job putting this extremely slippery, volatile and nebulous subject into context.
It's not a necessary text for those who want to experience Taoism. In fact, all the history and interplay with the other rulers, religions and rebellions might be distracting. Like reading about the life and upbringing of Jean-Claude Killy won't make you a better skier.
That said, it's a good resource for those who want to talk intelligently about Taoism to someone else. If you want to understand how Taoist philosophy relates to elements like alchemy, Buddhism, Confucianism, immortality, Yellow Turbans, magic mushrooms, Celestial Emperors, and even the Taoist Church, then this book is essential reading. Ditto if you are already versed in the Tao Te Ching and Chuang Tzu and just want to explore the history and influences behind the philosophy.
Lao Tzu would likely not approve, but I do. I've never dog-eared so many pages in a book, underlined as many words or starred as many paragraphs as I have in this book.