When critically evaluating sources it is good to remember OPVL and POOP.
O--Origin: Who wrote this source? Who are they? When did they write it? Where did they write it?
P--Purpose: Why did they write it? What is their agenda? Why write another book on this topic?
V--Value: What is the value of this source for my current research or investigation?
L--Limitations: What are the limitations of this source for my current research or investigation?
P--Primary or secondary source?
O--Objective or does the author have a reason to lie?
O--Other sources that corroborate or back up this author's point of view?
P--Professional historian or not?
Some research links: