So far, everything in this book has been in the key of C. After C, one of the next most common keys is the key of G. Here is the G major scale spanning two octaves in the treble staff. If you have trouble reading the high notes, take the scale slowly and name each note as you play it. Be careful with the crossover at the change of octaves.
To play in the key of G you will need to know the D major chord which contains the notes D, F# and A.