ISI Nyquist criterion and spectral efficiency
I don't have Sklar's text but I guess he is referring to the baseband case. For bandpass signals the required bandwidth is doubled. So in the bandpass case you get a maximum spectral efficiency of 1 bit/s/Hz for BPSK, and $\log M$ bits/s/Hz for $M$-ary QAM.

Just as a side note, spectral efficiency is measured in bits/s/Hz (and not in bits/Hz as you seem to suggest).