Some problem with the expression of QAM basis and signal
Base choices are ambiguous. The $-$ is there because the author of the book you're reading decided it was better that way.

You'll probably find the cause for that in the quadrature mixer graphic that they use, where if the phase shift from $I$ to $Q$ is $+\frac\pi2$, since $\cos(x+\frac\pi2) = -\sin(x)$.