There is nothing irrelevant in Sultan Khan book. Any one appearing for Public administration must go through it. Last time out it was almost a complete book but this time a few extra topics are there which are not in the book.
Keep syllabus infront of you and go through all the topic which are there in the book. For the rest Google.

Public admin 65 (2015 attempt) only studies SK book.

This time again kept it and would be following above mentioned policy.