This book is less a manifesto than it is a very interesting look at how architecture has evolved over the last 2000 years. Venturi evocatively shows that there was no straight line approach to architecture, but rather an ever-changing and ambiguous path that Modernists chose to make short cuts through. In this sense, Venturi really does capture the complexity and contradiction in architecture in that there are many lessons to be learned, making this book as valuable today as it was in 1966 when it first appeared.
Being one of the early "gray" architects, Venturi inspired a movement that eventually became characterized as "Post Modern." His early architectural work left a lot to be desired, since it seems less inspired by the many historical examples he favored, like Frank Furness, in this book and more by the banal trends in contemporary architecture at the time, eventually leading to Learning from Las Vegas (1972), where the concept of a building being a "duck," or a decorated shed, emerged.
This book's most appealing aspect is that it is immediately accessible. You don't have to be an architect to understand where Venturi is coming from, much less a grad student working on a dissertation. Venturi avoids all that senseless jargon that characterized architectural theory at the time and later came to engulf Po-Mo talk as well.