By making use of learned treatises, you can add to your stable of experts highly-credentialed writers who will testify for you for free. As Henry G. Miller puts it in his book On Trial: Lessons from a Lifetime in the Courtroom, "You don't even have to take anybody to dinner."
In some jurisdictions, the expert can try to frustrate the cross-examiner by refusing to concede the treatise is authoritative . . . Trot out the same ten books and the same meaningful looks."You don't admit the Einstein is an expert on the theory of relativity?"
"Did you know that Professor Gallen is Chair of the Department of Solar System Medicine at Harvard--Yale--Johns Hopkins--Mayo Clinic? You don't admit he's an authority?"
"You admit that you have Professor Galileo's book in your library? Isn't he an authority?"
"You admit you were taught by Isaac Newton at Princeton and you still deny he's an authority?"
"You say you are the only authority in your field?"
And so on.