The eagle and lark in the title are from a parable given at the beginning of the book. According to the parable, the lark wants to sing to the gods, but can't fly high enough, so he enlists the help of the eagle, who can fly higher than any bird, and together they visit the gods and the lark sings to them. The eagle represents reason, the lark intuition, and both are essential in the practice of astrology. The book, of course, can only help with the reason and technique part; on the other hand, you probably don't need help with intuition, though you may think you do.
The book gives solid explanations of both transits and progressions, how they operate, and how to predict their effects. A powerful tool given in the book is called a time map, and is a graph of transits and progressions during a time period. The time map uses another tool called the transit grid, which lists the transit type and the houses (which represent areas of life) that will show the causes of the related events, the place where the main action occurs, and the areas that will be affected as a final result.
This book isn't for beginners (they should read Astrology for Yourself: How to Understand And Interpret Your Own Birth Chart