Riding posture is determined by the bike and you. The bike will dictate some things about your posture. For example a sports bike will make you lean forward due to the bars being lower.
The approach you take should be the same no matter what type of bike it is. Overall you should be as relaxed as possible. There is no point in putting undue effort just to sit on the bike – you will tire very quickly. The following is for going in a straight line road riding, not on the track.
Going from the top, your head and eyes should be up to allow you to look as far ahead as possible. Your shoulders should be relaxed – tension in your shoulders will affect everything you do. Your arms should have a slight bend in them – it allows you to have some suspension in your body. Similarly the elbows should not be raised out or pulled in - no effort.
Your hands should rest on the bars so that if you drew a line through your lower arm it would go through the middle of the handlebar. The clutch and front brake levers should be setup so they are in the same line as your arms. This means you do not have to raise or lower your fingers / wrists to use them.
Your back should be relatively straight but if you’re most relaxed position is to slouch a little then so be it. Your “back side” should be as far forward as practical – close as possible to the tank without being uncomfortable.
Your knees should rest against the tank whilst riding along. Make sure they do not flap out but also you do not use too much effort to keep them against the tank. Save the effort of using your knees when braking. The arch of the feet should be over the footrests.
The feet should also be turned slightly out from the controls so you do not accidentally rest on them. The weight on the pegs should also be relaxed – the same as when sitting on a chair and your feet are on the ground.
The book will expand on these issues.