Your search for "William Cavendish" returned 2 letters.
Bess (dowager countess of Shrewsbury) writes to William Cecil, Lord Burghley, in reply to his letter about rumoured plans to abduct Bess and her grand-daughter, Arbella Stuart. Bess assures him that she will protect Arbella to the best of her ability. She also tells him about one of Arbella's attendants, Morley, who she has dismissed due to his suspicious behaviour. The letter was penned by her son, William Cavendish, because her head hurts; however, no one else knows of the matter.
William Cecil, Lord Burghley, writes to Bess (dowager countess of Shrewsbury), commending the marriage between Bess's granddaughter, 'Mistress Pierrepont', and George Manners. He also advises her to 'visit your friends and children, and not live so solitary as it seems you do there in Chatsworth amongst hills and rocks of stones'. This letter is a copy sent from Bess, via her son William Cavendish, to John Manners. William writes that once Manners has perused the copy, Bess desires him to 'return it again'.