Complex sentence: Writer Christopher Isherwood's collection is considered the most important private collection of his work; in the 1990s, Isherwood's long-time partner Don Bachardy donated the collection to a foundation.
Split into simpler sentences: Writer Christopher Isherwood's collection is considered the most important private collection of his work. In the 1990s, Isherwood's long-time partner Don Bachardy donated the collection to a foundation.