Tolkeins "Middle Earth" was divided into 3 ages. Lord of the Rings and the Hobbit recounted tales from the second/third age if memory serves, whereas the Silmarillion looked at events from creation and the first age. So, Chronologically speaking, the Silmarillion would be the first book. However, the first book that Tolkein wrote about Middle Earth was "The Hobbit", published in 1937. The Silmarillion wasnt published until the late '70s.
So, the answer to your question as worded above could be either "The Silmarillion" or "The Hobbit" :)