I think this is an important question and hope this is allowed here - so I will impose a more specific question. For example, if I am interested in learning more on George Washington, lets say I first read about him in my undergrad textbook, and secondly I read about him from a book I purchased off the shelf of my local book store.
What is the difference in the information I receive in both sources? Can I trust both sources equally? More importantly, are there any subjects in which the information will be more accurate from one source versus the other?
I’ll answer this in the context of academic rigor in books in general. Roughly speaking, textbooks tend to be pretty general in their presentation of a topic that in reality often have deeper disagreements amongst specialists studying that particular topic, person, event, idea, etc. Having said that, disagreements about facts are less likely than reinterpretation, revisionism or reframing of facts unless there has been a recent discovery of newer primary sources to add to the discussion.
Determining where a book on a topic falls on the spectrum of academic rigor and thus its reliability can seem a little tricky. I have my own subjective criteria that nonetheless works for me. Generally, those printed by a reputable university press are pretty fair and objective and almost always reliable. Those printed for mass consumption may be a hit or miss in terms of rigor. If the authors face is on the cover, it’s probably crap. Journal articles are probably the most academic.