Bb7(13)?

Ok, I was trying to figure out the chord tones for this chord, so I began by writing out the scale of Bb:
Bb C D Eb F G A - right? right...

from this the tones for the chord would be - Bb, F, A, G (assuming the chord consists of the root, fifth, seventh and thirteenth). However when I checked the chord in a chord book, it said that this chord contains a G#, not a G. What gives? What am I not seeing?

have a look at this site:
http://www.guitarnotes.com/guitar/Links/notes.cgi?chord_faq

It will teach you everything about formation of chords, starting from intervals and simple triads, up to weird ones like A13b5b9 :-)
Tell me what you think about it.....

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.

Oh, ok so that's what I was doing, forgetting to flat the seventh. Thanks guys