This bookplate is for John and Harold Boldero. John Boldero was a businessman. His son, Sir Harold Esmond Arnison Boldero (1889-1960), was Dean of the Medical School at Middlesex Hospital where the medical library was named after him, and was Registrar of the Royal College of Physicians. The library is now a part of UCL Library. Harold married a woman named Margery Dunn in 1917.
Balder is a Norse god who had a dream of his own death. The swastika is a symbol used by a number of religions including Hinduism, Buddhism, Jainism and Christianity. The leaves on the left are mistletoe.
Bookplate (click to enlarge)