In my experience, we usually think of the cycle of the cosine, like the sine, as going from 0 to 2\pi. The graph just starts at 1 and comes down, rather than starting at 0 and rising.
But as MarkFL said, you could consider a cycle to start wherever you like. If you were doing this for a teacher (or if you want to compare your answers to the book's), you have to check how they use the term.
