So, I've reached a spoken language topic and encountered difficulties with understanding those particles that are used in spoken language. Particularly with の. My book says that, for example, の combined with よ (部屋の中には暖かいのよ) means that a person is completely sure about their statement. I don't really understand the purpose of の itself in this case but I guess I just have to remember this so it's not really a problem, even though I'd like to know の's function here.
In an interrogative question the function of の is crystal clear so it's not a problem, I guess.
The problem is in understanding its meaning/function in the following sentence:
The book says that の is just a short form of のです but gives zero explanations on this one, that's why I'm asking you for help. I'd like to know its function and semantic purpose, and, if possible, I'd like you to give me literal translation of the sentence.
P.S. I apologize for creating this topic, because it already might exist here but I couldn't find any info about my specific situation (maybe my English is just a bit limited to understand some of the answers).