I'm trying to figure at the nuance between 学校まで1時間も掛かります。 and 学校まで1時間掛かります。 Book says it's used to express something plentyfull, but I'm not sure what exactly that means. What is the fine difference ?
When used with quantities, も is used to add emphasis and/or emotion, whether in a positive or negative sense.
As such it can often be translated to mean “as much as”, “as many as” with a positive verb; or “as little as”, “as few as” with a negative verb.
For example, compare the following:
I received 10,000 yen. (neutral statement of fact)
I received (as much as) 10,000 yen. (10,000 yen is a lot!)
It cost 10,000 yen. (neutral statement of fact)
It cost (as much as) 10,000 yen. (expensive!!)
Note that both sentences were positive verb sentences, but the emotional emphasis was positive when receiving and negative when paying. The emphasis/emotion conveyed depends thus on the context.
In your sentence, therefore:
It takes an hour to (get to) school. (neutral statement of fact)
It takes (as much as) an hour to (get to) school. (Likely expressing frustration that an hour is a long time to commute each way!)
Does that clarify it for you? I would recommend Maggie Sensei’s post on this topic, from where those examples were drawn, if you want to look at a more in-depth explanation. Hope that helps!