Best book to improve my English vocabulary

It is hard to understand when native speaker cracks a joke I face this awkward situation whenever i talk to native speaker. They crack a joke and here i am figuring out what to reply, cause I didn't get it. What should i do in that kind of situation. I can't ask them what does that mean, it makes me look stupid

Is it important to be grammatically correct in order to talk with one who is a native speaker Usually when i talk to a native English speaker and when they find out that i am not good at it, they stop taking to me. I know my english is not that good but good enough to carry on a conversation but still i found that many people left the conversation. I don't know what are the possible reason to left the conversation. While talking i usually ask questions and answer questions asked by them. Does asking a lot of questions affected my conversation, does it bother them. But it doesn't affect any of my conversation while Talking to those who are learning english too. I am just curious to know, why i am facing this problem. Any kind of help will be appreciated