The obvious second book to read after Laozi is Zhuangzi (Chuang-Tzu). His book is a joy to read, full of humor and anecdotes, where Laozi's reads like cryptic poetry. Both books describe a common worldview of harmonious living with the natural world, from complementary perspectives.
I personally recommend Burton Watson's translation of Zhuangzi.
I personally recommend Burton Watson's translation of Zhuangzi.