There are two books in Greek language instruction, "Greek: an Intensive Course," and Clyde Pharr's "Homeric Greek." All others are mere pretenders. Classical Greek is not an easy language for the English speaker, and recent attempts to flatten the steepness of the learning curve somehow fail to achieve to critical velocity in my opinion. A sturdy foundation in Greek that will bring a joy lasting a lifetime begins with some dedicated effort, and this book admittedly demands your attention if you want to be successful. Hansen presents the fundamentals of Greek grammar and morphology in a systematic and logical way, building up the foundation on which will stand the demands of more advanced texts.
I do, however, want to stress that this is an intermediate level book, and perhaps should not be the first book one attempts. Its 21 Units cover a great deal of material. Although, no prerequisite knowledge on the part of the reader is assumed by the text, it's explanations are very to the point and concise, which may not be as clear to the novitiate as needs to be. This book does not do any hand-holding to guide the reader through the difficult sections, so to speak. I suggest Clyde Pharr's "Homeric Greek" for the pure beginner, and then this book.
This book will reward you with the effort you put into it.
My only complaint is that it tends to ignore the arts in favor of the philosophy of the Greek culture. I would like to see more Sophocles, Euripides, and other playwrights of the Golden Age of Classical Greece. Language and culture exist together in a symbiosis, and the richness of the language can only be seen in readings from all the major genres, poetry, drama, history, and politics--not just philosophy, and certainly not just one author--namele Plato.