These letters sent from Katherine to her sister Rachel Cavendish begin before Catherine was married up until two years before her death in 1711. The topics of the letters include: Lady Cavendish being at Petworth; arrangements to see each other; sad news of Lady Julian from Mr Woolman; the rapidly failing eyesight of Lady Russell and Doctor Ridgley's failure to do much about it; Lady Alington's visit from Lady Cavendish; wishes from her aunts who are with Catherine and Aunt Mar's wish for Lady Cavendish to finish her picture for her; Catherine's displeasure at lack of word from her sister; requesting news of Lady Northumberland's visit; doing her mother's will; keeping out of some business to do with their mother; meeting at Chatsworth; concern for Lady Cavendish having many health complaints; her enjoyment of Lady Leveson's company and her wish to stay together over winter depending on Sir John's movements; a pad received from Catherine's father-in-law the Duke of Rutland; horseriding; a descripition of a saddle given to Catherine by Lady Rutland; mention of a gentleman met in a coach [Mr Thurwood?] that Catherine is socialising with along with Lady Thurwood and Lady Brownlow; wishing to see the new countess; Lady Cavendish and Admiral Russell's tendency to enlarge on things told; an incident concerning Mr Manners and someone who ran away with some of his money; Lady Bridget and Mrs Noel leaving London; employing a servant [or teacher] recommended by Mr Purcell and Dr Blond but rejected by the Duke of Rutland for being a papist; requests to know who Lady Cavendish's mysterious company in St James's park was; approval of a maid suggested by their mother; Mrs [Motun]'s ill dancing; the reliability of the post and Catherine keeping the letter sent to her secret; Betty Drew writing to Madam Dorothy; Lady Shaftesbuy's gout; cousin [Crones] being ill; travelling to Haddon [Hall]; increasing of Lady Cavendish's honours; visiting Woburn; improvement in Lady Russell's eyes; impassable ways in Derbyshire during winter; arranging their journeys for the winter; Lady Russell's intention to go to Southampton House; Catherine's desire for Lady Hartington's opinion on a matter; Lady Hartington's brother Harry and his loss of the business; Catherine's cold and a visitor who stayed during her illness and has many stories to tell; a story about Lady Rutland going to the inn at Levenhome; request for music and songs from Lord Harry and poems to be sent in the box to Catherine; Lady Hartington's pregnancy sickness; Catherine's advice for Lady Hartington concerning her pregnancy; Lord Harry's wedding; Catherine's pleasure at Lady Hartington's good friendship with Sir James; highest diversions performing below stairs; the countess being in a bad way; her happiness at the news that Lady Hartington has had a son; Catherine's son Tom and daughter Rachel recovering from their fevers; Lord Rutland's birthday party; visitors including Lady Irwin; Lord Granby in Lincoln for the elections; called to picket; an invitation to Chatsworth and requiring her sister to send a coach as she has reduced her horses to four and no coach; Mr Charleton and Lord Rayelton aiding in some business of Catherine's; some bills from Lord Granby to be received; sending documents in a box with Lord Orford if necessary; work for Nicholas Allatson in Lady Devonshire's family or Lord James in husbandry or the granary.
There is also one letter from Catherine to her mother Lady Russell (CS1/66/9) in which she writes of her gladness at her sister's delivery and also about her concerns for her brother (Lord Tavistock) in Rome in his familiarity with the women and his time away from his companion Mr Sherrard. She asks her mother to help discreetly in the matter.