I haven't seen the pre-G1 book, but if it's the same format as the graded books, then there are rhythmic exercises (which you sing/clap) and melodic patterns in addition to the actual sight reading exercises. I think these can be practised repeatedly since keeping steady pulse/rhythm and recognizing patterns are important sight reading skills.
However, I don't think going over and over the sight reading exercises themselves would help improve your skill much; once you've played the pieces a few times, you don't really "sight" read anymore so better find something fresh. There are lots of sight reading books in the market, or borrow from the library (if there's a good one near you), or simply print something off the internet. You don't have to always stick to pre-G1 pieces either; for pieces beyond your ability, just try one bar (or even one beat) at a time, hand separately (or, say, if it's a chord, play just the top or bottom note); at this stage, you aren't trying to actually play those pieces but simply spotting the notes anyway.
I think there is a recent thread on sight reading here in Viva Piano; lots of good advice there if you haven't seen.