Wouldn't that be the same as Bb7 add 13? If so, I would think that the chord would be: Root, 3rd, 5th, b7th, 13th

Using the Bb major scale: (Bb, C, D, Eb, F, G, A) the chord tones should be: Bb, D, F, Ab, G. That's where your book is getting the G# from. (G#/Ab). But noting it as G# instead of Ab is technically incorrect. G# is not part of the Bb scale. And you would not raise the 6th a half step to get the Bb7th chord. You would lower the 7th.