Oxford University's library has bought a collection of letters that shed light on Jewish writer Franz Kafka's personal life.

The collection, sold to the Bodleian for an undisclosed sum this week includes more than 100 letters and colour postcards sent by Kafka to his youngest sister Ottla in the early 20th century.

Preserved for decades by Ottla and her children, the letters are thought to make up the majority of the surviving correspondence between the siblings.

There are also more than 30 letters from Kafka's lover Dora Diamant and his friend Robert Klopstock.

The correspondence, some of which features drawings by Kafka, covers details of his relationships, his opinions on acquaintances, his diet and his understanding of the politics of the time.

The collection, which was already on loan to the library, was due to be auctioned in Germany later this month but the Bodleian and Marbach (the German Literary Archive) reached an agreement to purchase it jointly. Much of Kafka's archive is already in the Bodleian.

Richard Ovenden, the Bodleian's associate director and keeper of special collections, called the purchase "a cause for celebration for international scholarship".

He said: "The University of Oxford will strengthen its position as an important centre for studies of German and Jewish literature and culture."

He added: "Our collaboration with Marbach will go beyond the actual purchase and extend to a partnership with a programme of exchanges of academic fellows and exhibitions as well as other research activities related to Kafka and beyond. It promises a new, exciting departure in Kafka studies."

Oxford German Professor Ritchie Robertson said the letters represented "an essential source both for Kafka's biography and for that of Ottla David, nee Kafka, an unusual and courageous woman whose efforts at emancipation have never been fully recognised.

"The letters need to be accessible at one or both of the principal sites for research into Kafka's literary manuscripts and personal writings."

Considered the founding father of literary modernism, the Prague-born writer's work included the novel The Trial and the short story Metamorphosis. He died of tuberculosis in 1924.

Other Kafka manuscripts are the subject of an ongoing legal battle between the National Library in Jerusalem and Eve Hoffe, daughter of the secretary of Kafka's friend Max Brod. In July, boxes of his work hidden for 50 years in a Swiss bank vault were opened to help lawyers determine where ownership lay.