The previous poster gives some good advice. The only thing I'll add is that books published by well known publishers are generally safer than those that are not. But different publishers all have different reputations. Anything published by a university carries the prestige of that university, so a Harvard, Yale, Oxford, or Cambridge book is pretty safe. But beyond those famous publishers, there are other university presses which carve out little niches for themselves, or do good work without the huge name recognition. University of Kansas Press is pretty fantastic, even though you wouldn't think that at first. "Popular" presses, like Random House or Penguin are a little more tricky. While a University press will usually peer review a book before running it, a mass market press doesnt have such obligations. Theyre more interested in the profit, and will sometimes publish salacious books. That doesnt automatically mean everything thats published by a mass market press, or for the mass market generally, is bad. Far from it. There are a number of mass market "pop" history books which are great, but its usually a warning sign. It also usually is accompanied by other warning signs, like a lack of notes, bibliographies, or citations. Or even if they do have citations, questionable sources might have a number of "bogus" citations. Think websites youve never heard, opinion pieces cited as fact, books and articles all written by the same author, that kind of thing.

Checking reviews is a good starting point, but there are other metrics you can check. Who is the author? Does s/he have a degree of some sort in a field related to the subject? Does the author provide sources for arguments made in the book? Does the book have arguments? Are there footnotes? Endnotes? Is there a bibliography at the end? Is the source recognized as legitimate in its field? What is the publisher? If it is attached to a University, it is probably good.

A good way to investigate a source is to do a search on a University's library system (this is usually accessible by anyone). For instance, search your book on the League of Nations on Harvard's HOLLIS system. Does it show up there? If it does not, it might suggest the source is not a very good one. Not every library has every book, of course, but the major University libraries will have most of them, especially on topics like the League of Nations. And as an aside: I can promise you that the League of Nations has indeed been covered extensively. There are probably hundreds, if not thousands of articles and books on the subject!