The direct translation is too confusing. In English it would be "I read books BY the million(s"). If it is to mean "I read books like millions of others" it should say so, but would be a complicated translation? I suppose this is why translation is such a skill. Not a good choice for beginners!
The equivalent idiom in English would be, "I read books by the dozen". The meaning of idioms will often be difficult to communicate in a language course. Just knowing the literal translation will give no clue as to how you would use such a phrase. Adding a short explanatory note might be really useful for situations like this.