Though the brother of one of the most famous poets in the English language, it was not suspected that Robert Sidney had himself been a poet until the 1960s, when his working notebook emerged (in a 19th-century binding) through the dispersal of the Library of Warwick Castle. Subsequent research showed it had been acquired in 1848 after passing through a number of sales beginning with the dispersal of the library at Penshurst in the early 19th century. Sold again at Sotheby's and acquired by the British Library in 1975 (catalogued as Add MS 58435), the autograph is, as its first editor P. J. Croft pointed out, "the largest body of verse to have survived from the Elizabethan period in a text entirely set down by the poet himself". Dating apparently from the latter half of the 1590s when Robert Sidney was governor of Flushing, the collection comprises 66 sonnets, songs, pastorals, elegies and slighter pieces, apparently structured as a kind of reply to Philip Sidney's "Astrophel and Stella". They show Robert Sidney as an advocate of the Neo-platonic philosophy of love and adept at a great variety of verse forms. The fact that several of the poems are based on identifiable tunes confirms his interest in music. While he cannot be placed in the first rank of Elizabethan poets, his poems are by no means negligible and of the greatest interest for the working methods and intellectual interests of the period.