How do you read a book and trust in it?

Practice source analysis: ask who wrote this and why. Consider where the author was born, raised, educated, their socioeconomic background, family connections, and institutional affiliations.

Read widely. Seek out authors that disagree and read them. Sometimes the truth lies with one side, sometimes it lies in a synthesis and fusion of various sides. There are often as many sides as there are experts in the field.

Consider the arguments made. Are the points made evidence-based or ideology-based? Some ideological frameworks are good guesses in the absence of evidence (and there is sadly all to often inadequate or imperfect evidence), but don't be afraid to overturn or carve out exceptions to an ideological statement if the evidence contradicts it. History is messy.

Do I have to adjust and go in with the mindset of "what I may come to know as a fact may be wrong in a few years."

The moment you cease to question what you know and how you know it you cease to practice science. Provisional beliefs cultivate a curious mind.

It's important to understand that sources and other forms of evidence determine the extent of what we know about any time or event or person. Many many books have been written and entire political or religious movements develop and argue about what "truth" constitutes. Some things and events are unknowable because there just isn't enough evidence to show us one way or another. So, no one knows what Christopher Columbus, William Wallace or Robert Hooke looked like. No one can know who fired first at Lexington, or how Harold died at Hastings, we don't know how many of the Roman Kings were actual individuals. But that does not mean nothing is knowable even with distant periods like Rome. We can make safe assumptions that most documents that survive from that period at least have some element of truth to them. People like Livy and Polybius, or Cato left us detailed accounts about events they witnessed or past events that they studied with evidence lost to modern times. We must always relay their reports with some skepticism but there is no reason to entirely discount them. The best thing to do is to read more and look at the footnotes that explain where a writer draws their conclusions from. If you would like a basic intro to the Roman period including an explanation of its limited sources I suggest SPQR by Mary Beard.

There are academic fads. History seems to be somewhat less subject to them than other humanities, but they still exist. You will get a very different view of a period of history reading two books written 30 years apart. The nature of history is that authors have to choose what to include and what to exclude.

But like science -- nothing is fixed. You have to be ready to drop a theory/fact on the basis of evidence.

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Some first filters:

• Avoid books that don't cite sources. There should at *least* be a bibliography per chapter, or a chapter based, "further reading"
• It's ok to read popularized accounts. Take with a grain of salt, but they can give you a quick overview.
• Look at the qualifications of the author. Erich von Daniken is not considered a reputable source. There is an engineer out who has a theory that the Homerian tales are transplanted from the Baltic region by migration. It's getting some interest by historians/classics scholars but is far from accepted at this time. That said, new theories are often put forward by people outside the field. But understand standard views first.
• Read reviews of the books you are considering reading. This will give you some idea if other historians regard the book as a good one.
• If a book has had multiple printings it can mean it was popular. But popularity can be either from sensationalism, or from being well regarded by other historians. In any case it's had a chance for errors to be corrected. Points for a book that has been reprinted.
• It's a good idea to make notes. This seems like a waste of time for a hobbyist but even noting the chief ideas on a chapter by chapter basis will let you find things later.

Along with what others have said it's a good idea to read up on the historiography of the topic your book/show covers. There can be a number of ways to view particular period of history and views do change over time.