Honestly, I recommend you go back and carefully re-watch the video again. Writing variables should become second nature to you in programming, so make sure you understand what a variable is, and the syntax for declaring one.

To answer your question though:

string bookTitle;

Again please go back and re-watch the video, take notes if you need to. For future advice, if you watch a video about a specific topic, and then need to perform a challenge for that topic. If you are unable to recall how to implement said topic, then first re-watch the video, before asking for help, you may just understand it better a second time.