From where I now work I find that I arrive far too early for Orchestra rehearsal and I`ll have 15-20 minutes to myself to read.

Lately I`ve stopped reading books and have brought in some technical books and have been practicing them in the time before rehearsal actually starts. I started with simple low range books which really were nice melodies so as not to draw too much attention, but occaisionally you hit one of those wonderful bits full 16th note runs all over the place and its really fun when you can play them really fast and accurate... it`s when I hit one of those that I get looks from people.

I`m also new to this Orchestra...

Is it considered rude to practice your own technical books in Orchestra rehearsal? I`m afraid I may appear to be showing off. Is it normal in other Orchestras for people to practice?

That is fine. Everyone chooses different ways to warm up. If it is working for you, then that is what is important.
The only thing that is considered a no, no, is if you are on second flute and you are practicing the first flutes solos or music. Other than that, your okay.