If you find you are scoring in that range, I do agree that you are probably just making a few silly errors rather than not knowing the content. Most books cover the content pretty well, but if you find that you keep making errors, make a word document and scrutinize every question you miss from practice, no matter how silly it is. Write down exactly what was going through your mind at that time which led to the incorrect answer and what you should have done to give you the correct answer. Make sure to go over this doc multiple times.
I did this and brought my ACT section scores in English, Reading, and Science from low 30s/high 20s to 36s in all when I took the real thing with little prep other than scrutinizing my mistakes.