Recently, I just finished Michael Hartl's rails tutorial. Throughout the book he recommended The Rails 3 Way after you finished his book. From your experience do you think it would be better to read another book or start building an app and figuring out things on my own?
People generally retain information better when they apply what they have learned. If you don't practice what you've learned then you risk losing knowledge from your mental stack.
Unless the book you read was perfect and you have photographic memory you will find yourself having to go back and reference that material and Googling for items that were not covered.
Practice what you've learned. And practice as you go. And don't make one-or-other decision. Why not read a book AND develop something at the same time? The two activities do not have to be mutually exclusive.