when do you use the singular and when do use a plural noun ,before the word "list"?

I would use the singular noun because 'list' explains you mean a collection of them.

But I think that sounds clumsy. Better would be a 'list of books'.

Funny, before the word 'list' it's singular, but after it is plural. That sounds funny, but 'books list' sounds funny too.

The word list means a series of a certain thing, such as books or trails. Therefore, the plural is already indicated in the word, list. So any word you place before list would be singular, as in book list or trail list.

It is usual to say "book list"; "trail list" etc. The dominant noun is "list", and "book" (etc) is used as though it were an adjective to describe the list, so it agrees in number with the singular "list"