One important thing to remember is that generally no single book will give you complete knowledge of a subject. Every author has their own views and biases; perhaps they're even promoting a thesis of theirs which hasn't been completely fleshed out. For that reason, I'd advise to always read multiple books on a subject. Three seems to be a good number unless you really want to dive into something.

For instance, Geoffrey Parker's book The Military Revolution is quite a good read, but it also overreaches. Reading Jeremy Black's War and the World would help you realise that. Add John Keegan's History of Warfare to it for another view of the development of warfare over time and by then you have a basic but decent understanding of the development of warfare.

Similarly, if the topic is Roman expansion, William Harris' War and Imperialism in Republican Rome puts forth a compelling - but not exactly right - thesis. Arthur Eckstein's Senate and General builds on that thesis and adds to it. John Rich (ed.) tries to complete the picture in War and Society in the Roman World. Again, after reading these books you'll know not only quite a bit about Roman expansion, but also about the various ways it can be approached.

This applies to pretty much any topic. For the Dutch Republic I always say you only have to read Jonathan Israel's The Dutch Republic, but that's a lie. If you want to know more about the culture of the Dutch Republic you'll also have to read something like Simon Schama's The Embarrassment of Riches.

So my advice to you is: once you've found a topic you're interested in, try to identify a few (i.e. more than one) influential works in that subject and read those. Try getting conflicting views* if you can, it'll help your understanding of the subject.

*That is 'conflicting views within the mainstream'. The discussion will focus on details or on interpretations of facts/developments. Don't read nonsense just because it contradicts established history.

/u/Polybios has good advice here. I would also recommend that every time you pick up something to read that you check the internet for evaluations of the work. Don't trust anything you read on the internet - of course - but it might give you an idea about what people regard as the strengths and weaknesses of each source.

And post a question to this sub: "I am about to read XXXX. Can anyone tell me about its flaws, what its author is attempting to do, etc.?"

A historian in a previous century once told me that a good part of being educated in history is knowing what sources are standard to read and to cite for a given topic. Another one of my teachers was keen to drive home the need for source criticism, and this goes for primary and secondary sources: always ask about context, bias, flaws, etc.

I would also suggest a few things that might help get some better footing into the field.

If you have time, you might consider enrolling for free in one of the Coursera history courses online, offered by some of the best institutions in the world.

A couple tips to get an idea of how academics approach history:

For subject areas that you're new to, what helps to identify important sources is the number of citations a certain book/article has, which you can check on Google Scholar. For instance, if I type in "vietnam war and media", the top result will be Dan Hallin's key book on the subject. Try reading those key source books first, and also look through the citations to see how the scholarship reacts to the book/article it over time. Note that this is not a fail-proof method, but it is generally helpful.

To understand where academia stands on a certain subject area today, the best thing to do would be to pick up or browse through articles on the subject, published in relatively recent copies of the best journals for that specialization. Usually you can identify these journals through their affiliations to leading academic historical associations, or by reading the footnotes and/or references on already reputable sources. JSTOR, which carry a lot of these journals in digitized form, will give you some free access to articles if you sign up.

Good luck in your studies!