Freshwater Leeches of Britain and Ireland by Elliot and Dobson, available from the Freshwater Biological Association

https://www.fba.org.uk/shop/p/book-43

Guide to the freshwater leeches of Britain and Ireland by Macadam and Davies, available from the Field Studies Council

https://www.field-studies-council.org/shop/publications/freshwater-leeches/