Correct footnote:

There is no 'right' answer to this exercise. Your footnote above may look something like this:

Humphrey Mill, A Nights Search: Discovering the Nature and Condition of all Sorts of Night-Walkers; with their Associates (London: Richard Bishop, 1640), p. 5.

But it could also have looked like this:

Humphrey Mill, A Nights Search (London: Richard Bishop 1640), p. 5.

Or this (note the use of dots in square brackets where we omit material from a title):

Humphrey Mill, A Nights Search: Discovering the Nature and Condition of all Sorts of Night-Walkers [...] Together with Diverse Fearfull and Strange Accidents, Occasioned by such Ill Livers (London: Richard Bishop, 1640), p. 5.

Or this:

Humphrey Mill, A Nights Search: Discovering the Nature and Condition of all Sorts of Night-Walkers; with their Associates. As also, the Life and Death of Many of them. Together with Diverse Fearfull and Strange Accidents, Occasioned by such Ill Livers (London: Richard Bishop, 1640), p. 5.

Comment:

When dealing with long titles, it is up to you to judge how the text's title can be most effectively abbreviated while still being clear to your reader. You may find it useful to review other works of secondary criticism to see if there is a standard short-title used by other critics working in your field.