ときめく妖怪図鑑

This is a short introductory guide to youkai with lots of illustrations. It's broken into several parts - a history of how people thought of spirits in Japan, a brief look at about ~35 specific youkai (with gorgeous illustrations), including a couple Ainu and Ryuukyuu spirits, a few essays looking at specific themes like 'Buddhism and youkai', and a wrap-up with some pictures of youkai-related goods and books.

One thing I liked was that although the book takes a generally practical view of youkai (it teases a little bit of the 'but are they real?', but only a little), it's also emphatic to the historical people who truly believed in youkai. People who thought that echoes were an invisible creature shouting back at them weren't dumb; they were trying to find explanations for the strange world around them. It also explains some possible deeper meanings and metaphors behind some stories, such as representing the clash between rice farmers and iron-working peoples.

This book is targeted at adults. It uses some formal literary words and doesn't leave much in kana when kanji will do (おもちゃ is written as 玩具, カタログ gets written as 型録, which I can't even type in my IME). There is a little bit of furigana on rarer words. The book also throws a lot of historical figures with long names at you, sometimes in quick succession - I recognized some of them from other reading or listening to Sengoku Daimyo, but certainly not all of them. For me, that made it hard to follow some sections. I noticed a decent amount of repetition of new-to-me words; there were also a few words I struggled to look up (such as 理水).

My experience was that the front section was a little difficult, the youkai catalog varied a lot (but because each section was short, if it was too difficult or boring, it was easy to skim and move on to the next), and the essays at the back tended to be the hardest.