Using free handbook/guidebooks for decision making

Thousands of free reports, guidebooks, and handbooks are available on the internet if you know where to (and how to) search them.

I want to see if they are actually used by startup founders/managers; basically anyone involved in operations. Do you use them? If so, are there any specific websites where you are searching for those free resources? How to understand which resource to trust vs. not-trust?

Following is the example of handbook I am talking about - https://postclick.com/ebooks/mastering-google-advertising-for-d2c-brands/?country_dropdown=Georgia