It is *absolutely* necessary to make good friends with your textbook. You should know everything about it, where to find information and critical facts. You must become completely familiar with it – to annotate and develop it so completely that you have extracted every last microgram of potential knowledge out of it – you should be able to turn to any piece of information without consulting the table of contents or the index.
But that is not enough. Not nearly enough – because Physics is not an “open book” subject – you can’t bring the textbook into (most) assessment tasks. (p.s. this is something to be very thankful for – you will discover (in time) that “open book” is the nastiest thing that any teacher of any subject can do to you…). You need to understand the difference between active and passive knowledge – and you *need* to make your knowledge of physics *active*
Passive knowledge is the sort of understanding of a topic you have when an expert explains it to you, and you feel that it makes sense. Active knowledge is when you can explain the knowledge that you have to someone else in a way that they understand it – and that you can use the knowledge you have to solve problems and apply it to a variety of unfamiliar contexts.