Hallo!
I have studied that a noun modifying another noun must be connected with the particle の, as in 日本語の本.
However, I frequently find nouns together with no particle at all joining them. An example:
くりむしようかんです
(page 9)
If this is not a compound word (at least, it appears as three separate words in the dictionary), how come there are three nouns one after the other without any particle?
Thank you very much for your help.
Well, it’s the name of a specific food, so the rules are relaxed. I could call something an “eggplant parmesan sandwich” even though if I wanted to describe it I’d have to do more than just smush the words together - you know what I mean?
Also - むし isn’t exactly a noun here, it’s 蒸し, so it’s a bit like “steamed.” (From the wikipedia page it sounds like it’s the youkan that’s steamed, not the chestnuts, but I’m not 100% sure about that).
EDIT: (according to a more general wikipedia page, 蒸し羊羹 (むしようかん) is youkan prepared in a particular way that involves steaming, so this name derives from that with くり prepended)
くりむしようかん is the name of a specific sweet made of steamed youkan with chestnuts.
I think you could say either くりようかん or くりのようかん if you just wanted to talk about youkan with chestnuts or chestnut flavor in general. The version without the の would maybe seem a little more like a name, the version with の would maybe seem a little more like a description.
Like, “I’m baking chestnut cake!” vs. “I’m baking a cake with chestnuts!” if that makes sense?
Doesn’t mean you can just go around slapping nouns together instead of using の all the time! And I would indeed describe くりむしようかん as a “compound word.”
You’ll find the “nouns just being stuck together” thing mostly in names of things/places/organizations, stuff like that. In usual grammar if you want to describe things, you’d use の.
As explained above, this is a “proper noun” situation really. This means that there is no need for の particle between a noun and it’s modifier. I will attach a little fragment of the book (I hope I won’t be punished for doing it 
) Making Sense of Japanese Grammar: A Clear Guide Through Common Problems by Zeljko Cipris & Shoko Hamano (page 96).
As you can see, the proper nouns don’t take の particle. And indeed, you will find them in many situations, especially news, etc.
Slightly off-top here: You might push this topic a little further and ask, what are these na-nouns, why do they take な and what is the difference between them and no-nouns, since both categories describe nouns? Na-nouns are simply na-adjectives. You’re probably aware by now that they’re often addressed as nouns. When you look at the examples on the right side of the picture, it might seem like there’s not much of a difference between, say, “convenient” and “free”. Why do they say 戦争の時 (wartime), but 平和な時 (peacetime)? That’s because 戦争 is a specific state that needs specific conditions in order to happen, whereas 平和 is more general. It’s easy to say if there’s war out there, but even during peacetime bad events always happen, right? Same applies to 病気の学生 (sick student) and 健康な学生 (healthy student). Overall, の is used with a noun modifier when you refer to quantities, absoluteness, uniqueness, etc. It’s all explained in the book, I don’t take any credit for it.
Yeah, you can put together some pretty chonky compound nouns by just gluing individual nouns together, but these are almost exlusively used only in the names of specific things, for example 東北地方太平洋沖地震 is the name of the March 2011 earthquake in Tohoku, and 国際連合教育科学文化機関 is what UNESCO is called in kanji (though you’ll probably see it written as “UNESCO” or “ユネスコ”, or referred to obliquely with “世界遺産”).
For bonus ridiculousness, there’s a temple in Chiba whose full formal name is 三図河頭極楽東門蓮華台上阿弥陀坊太平埜山本実成院長福寿寺, and there’s a bus stop in Shizuoka named 曲金静岡視覚特別支援学校静鉄不動産静岡南店前.
Thanks, everybody.
You’ve helped me a lot.