The Book of Hallowe'en is about what you'd expect--or at least, it was exactly what I expected. It's basically a bunch of traditions from over Europe briefly explained, and a few from the USA, too. There's not a whole lot of depth, but there is a very long references section which can probably provide that depth. It seems to have been factual for it's time (around 1919), though the ways we view facts and history and primary sources have all changed, so I don't know how well it holds up nowadays as a factual document.
My complaints are few, but I'll mention them; first the lack of depth that I mentioned, which sometimes made me feel as though I was just reading a long list of anecdotes. Second, I'm not sure how to consider some of their sources. There were plenty of quotes from tradition songs, and then there were quotes from fiction like Peter Pan, and I couldn't tell if the author was analyzing the newer stuff to portray new depth, or if they were using fiction as a primary source, or what.
If you enjoy books like this, either in style or in topic, I also recommend
Folk Lore Superstitious Beliefs in the West of Scotland within This Century
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