Ruy Lopez -logic behind the moves.

This question / post comes from the book Strategy and Tactics in Chess by Dr. Euwe page 3. 

http://www.chess.com/groups/forumview/our-max-euwe-corner-ii

This link is to the Chess Hustler forum.  From there it shows you how to download the entire book.  The opening below is mentioned on page 3 and is Ruy Lopez [C82]. 

To this point, I wonder why black on move 4 delayed b6 and why on move 5 white castles rather than protecting e4?

What do you think? 

Do you play the book line?