Are you sure you understand the main meaning of the preposition "in"? It usually means "inside". On that basis, how can you have "books in your legs"?
Be careful with your spelling. Check all the words before you post. I am sure that you know that the word you needed at the end was "thanks", not "thinks".
Edit: While I was typing my response, you edited your post and added more at the end. Your understanding is mostly right but it's not an absolute rule. You will find a lot of exceptions. Please use the word "something", not "sth". I know that "sth" appears in dictionaries but they do that because there is not very much space. Here, you should type the complete word.
Student or Learner