We can change this sentence into a question tag as follows; You'd better read English, won't you?
What are question tags?
Question tags are those phrases that are inserted after a statement to reinforce an idea that was previously stated. The purpose of question tags is to help the reader confirm that what was said was true or false.
In the case above, the reason is to enable the recipient of the information to confirm whether they would read their English text or not. Other ways to add question tags are didn't you? can't you? Did she? isn't it? etc. So, in the sentence above, Won't you is most applicable.