Tao Te Ching is ancient, now a couple of millenia in print. Stephen Mitchell has not translated this classic, but rather has paraphrased it -- as he admits in the Foreward. But he is a Zen student of a couple of decades and has good insight into the Zen of the Tao (Zen Buddhism is Buddhism heavily dosed with Taoism).
Mitchell's version of the Tao Te Ching is very, even extremely, modern. Perhaps to the point of being "politically correct." However, he does have a way with words and this is a very readable version of the Tao. To show how modern it is, let's take an example and compare his version of the beginning of chapter 46 with two other versions:
"When a country is in harmony with the Tao,
the factories make trucks and tractors.
When a country goes counter to the Tao,
warheads are stockpiled outside the cities."
- Victor Mair
"When the Way prevails under heaven,
swift horses are relegated to fertilizing fields.
When the Way does not prevail under heaven,
war-horses breed in the suburbs."
- Addiss & Lombardo
"With TAO under heaven
Stray horses fertilze the fields.
Without TAO under heaven,
Warhorses are bred at the frontier."
Obviously, there were no factories, trucks, tractors, or warheads in ancient China. So, Mitchell is providing a modern interpretation of the Tao Te Ching, while Mair as well as Addiss & Lombardo are closer to a literal translation (which is not possible however, because the Chinese language and the English language are so completely different from one another.)
None of this is to find fault with Stephen Mitchell. This is just to say that his book cannot be definitive, because it is less literal and not really a translation. However it is good, compelling reading, and honestly makes no pretense of being a literal translation. If you like Mitchell's approach, get one of the more literal translations too. I bet Stephen Mitchell himself would like you to have both.