Eighteenth-Century British Premiers: Walpole to the Younger Pitt
Eighteenth-Century British Premiers: Walpole to the Younger Pitt (ISBN: 978-0230284784) was written by Dick Leonard in 2010.
I haven't actually read this book, just cited the first page: "Historians are agreed that Sir Robert Walpole should be accounted the first British Prime Minister, though he never held the formal title, and nor did any of this successors until Sir Henry Campbell-Bannerman, in 1906-08."