Most textbooks don’t cover all subjects equally well. Rather than read a textbook, the best approach is to read around the topics.
If a topic requires detailed knowledge, then you should aim to summarise everything which is contained in the reference texts. For good knowledge, you should read through the texts which cover the subject best. If it is basic, you will probably get by just learning from good coverage in one or perhaps two of the texts. You will need to look through them all though to find the best one.
The primary aims to assess understanding at a specialist level. You may find that the texts say different things. In this case you should try to understand why. It may be that one text is newer than another, it may be that they used different studies, or that the studies are contradictory.
If you are asked about the topic in the exam, rather than saying that the textbooks say different things, you can then explain why. If a topic is controversial, don’t be afraid to discuss what the disputed issues are.