The books by Starke, Brownlie or Agarwal and Tandon are good enough and most recommended so you don't need to go through Malcolm Shaw. Just stick with one of the former authors preferably Starke.
As far as thorough study is concerned, it's always good and I'll suggest you to do it however there definitely are some topics that you cannot afford to miss. A list of such topics is available at
this link so make sure you prepare them well.
You feel like sleeping while studying International Law!? Goodness!
All the best.