It's like Stabbey says. The higher the skill level, the more specialization is expected. You can still learn the skill, but you'll get an AP penalty for every point you "fall short".
So a skillbook will now show you the level of the skill. The higher this level, the more penalty you'll get if you have a low score. You will notice if there is a penalty on a skill because the description of both skill and skillbook will TELL you (in red I believe, under the AP cost line).