A keen historians first skill should be to acknowledge the fluid nature of our historical understanding and that no source is infallible. Always keep your wits about you when reading anything, and I mean ANYTHING, there has and always will be an agenda at play whether overt and intentional or covert and accidental.

Understand that even the sources you mention have their own biases in how and what they report, and that's fine! That is a natural facet of literature, it is inescapable; all the reader can do is try to bear in mind the following:

Who wrote this, and why?

What was/is their intended audience?

With modern secondary sources I also find it useful to consider whether they take their own word as gospel; if the author is authoritative in the extreme then I find them questionable. The thing you come to understand as you learn is that you understand nothing, all we can do is string together the evidence we can and make tentative judgements from there.

I hope this helped, unless you can afford to fork out for JSTOR then the advances of academic thought are somewhat obscure. I would happily take information about anywhere I can keep up with new ideas.