Translation:The leisurely old men read the long book.
This is bad English. "Leisurely" is an adverb, and in English, that would normally modify a verb, hence: "The old men leisurely read the long book. If this is not acceptable, then you'd say, "The old men of leisure read the long book."
"Leisurely" is also an adjective. However, it is usually used on nouns that refer to some 'action' (a leisurely lunch, a leisurely pace, etc.), so it does seem weird here (at least to me).