Good thread...Wikipedia is reliable, notwithstanding that in today's super-fast information age, there is a margin of error to everything you read. What was true yesterday, may be proven wrong tomorrow. But that's also why online material can adapt faster to changes. I'm all for paper-books, but to say that f.e Aristotle is more reliable on physics just coz his works are printed, than a wikipedia article, now that'd be stupid, right? And if his works are uploaded to an online library, what's the difference? Nothing. Outdated info is outdated, paper or not. Not to mention that those reliable books are usually cited on wikipedia, so you can check them out. If you find a factual mistake? Well, go edit it, anyone can do it after all, right? Of course if I don't find sufficient info on wikipedia, I go to other sources as well, but for a quick research, wikipedia is probably the best choice. Now, for doing school papers, you can't directly quote wikipedia, but you can always look up its cited works and quote those in your paper. However, databases like academia.edu, or google scholar are probably better if you are looking for studies in a specific field- but if you don't want to read (or you just don't have time to read) hundreds of studies, then wikipedia usually serves with a fair and unbiased summary. It always mentions arguments and their counters, the different opinions of different experts...etc. + scholars/Phds...etc can also edit wikipedia...
But again, noone said that wikipedia is the pinnacle of education...
Just one personal example tho...during my uni studies, we had a book on universal legal history that had literally no citations, no sources quoted, no bibliography...nothing. I still don't know how that book could pass. So no, not even uni books written by experts, are always reliable...Not to mention that an academic book authored and lectured by only one or a few people, may use many sources cited, but it's still limited in its scope simply because of the limited number of authors and the limited time and money they could put in their research. You also have to take into consideration the different purposes of a site like wikipedia, and a scientific book. The latter usually summarises the research of its author(s), while wikipedia collects the results of several such authors. Now ofc it's possible that a book is also published on the latest/contemporary results of a field of study, with the intent of giving a general overview. This is called meta-analysis, and it's different from when individual authors publish their own results. Note that wikipedia doesn't necessarily (I guess for most of the times it doesn't) lean towards one side or the other. That's not its purpose. While individual authors usually want to prove something is correct or not,f.e someone writes a book on the evidence for evolution, while someone else writes a book on why that's not true. Wikipedia may cite both -if the authors are relevant enough- but what wikipedia won't do, is to say that the guy who wrote the book on why evolution is fact undoubted, right, and the other guy is bs. Wikipedia may add that the majority of scientists say this or that, but that's all.
And then still there is the problem of being outdated. Sometimes it takes years to revise a book, and sometimes they don't bother correcting the outdated info when it comes to paper books, simply because it's not profitable anymore, but to my experience, wikipedia adapts faster.
Just my 2 cents.