Tostig Godwinson (~1026-25 September 1066), Earl of Northumbria, was one of the brothers of King 
Harold of England, its last Saxon king.  Tostig married Judith (Fausta) of Flanders (1030-5 March 1094), the half-sister of Count Baldwin V of Flanders and, thus, the aunt of 
Matilda of Flanders who was the wife of 
William the Conqueror.
 
Tostig had joined forces with King 
Harald Hardrada of Norway, and they invaded England just before William the Conqueror did.  Harold of England managed to defeat Harald of Norway at the 
Battle of Stamford Bridge on 25 September 1066, where Tostig was killed, but then Harold of England was himself killed at the 
Battle of Hastings on 14 October against William, who thus became King William I of England.
Popular (as opposed to scholarly) non-fiction books that cover Tostig's life and role in history include:
 
All Wikipedia text 
is available under the 
terms of the GNU Free Documentation License