Short answers, pulled from the story.
Relativity: The Special and the General Theory was first published in German in 1916 and translated into English in 1920. Abraham Pais records that the expanded book was completed by Einstein in December 1916.
Einstein wrote the book for readers interested in the theory from a general scientific and philosophical point of view who were not conversant with the mathematical apparatus of theoretical physics. He deliberately treated the empirical physical foundations in what he called a step-motherly fashion so that non-specialists would not be overwhelmed.
The book is divided into three parts: the first deals with special relativity, the second deals with general relativity, and the third deals with cosmology and considerations of the universe as a whole.
The book describes three key predictions: the 43-arc-seconds-per-century precession of Mercury's orbital ellipse, the curving of light near a massive body (confirmed by Arthur Eddington in 1919), and the red-shifting of light from a massive star (confirmed by Walter Sydney Adams in 1925).
Einstein uses a thought experiment involving lightning striking two points on a railway track. An observer on the embankment midway between the two strikes sees them as simultaneous, while an observer on a moving train does not. The book concludes that events simultaneous in one reference frame are not simultaneous in another, and every reference body has its own particular time.
Demand for the book rose sharply after the 1919 eclipse expedition results were announced. A review in Nature described it as an answer to the question thousands had been asking about what relativity was. Walther Rathenau wrote personally to Einstein saying he would not have thought it possible to force such a radical rearrangement of ideas through with such simple means.