A serviceman's military records, discharge papers, tickets for transport etc. from the Boer war, in a volume of Pope's letters.
A family tree in a 17th c. Bible, behind a pasted over frontispiece, going back hundreds of years, complete with a baby's handprint from the late 18th century.
War cabinet correspondence with an editor at the Times from WWI inside a BBC archives bound copy of the times from the quarter surrounding the outbreak of the war.
A £5 note - one of the old white fivers the size of a dinner-napkin, inside Dulac's Arabian Nights (one of my favourite purchases - £1 from a charity shop, first edition, gorgeous book).
A land deed for a farm in Wales from the 1680s, although this was in a Victorian desk-tidy, under the lining paper, and half eaten by woodworm.
All in all, about 5% of the books and objects I buy end up coming with some sort of history bonus.
On a larger sort of scale, my mother bought a house in southern France in the late 80's, and while doing renovation discovered a walled up loft containing a hoarde of art up to the early 1960's - we did eventually track down the rightful heir, who in gratitude for our discovery allowed us our pick of anything we wanted within reason. It had long been thought that his grandmother's husband had done a runner with the lot - turns out granny was just bitter and disinheriting his children after he ran off without the art.
Funny old world. Funny old humans.
Edit: just remembered another hoarde of mementos I found: under the floorboards of a Georgian building in Bath while laying network cable in our office. Found a Radio Times poster advertising Dame Nelly Melba on 2LO, the BBC's first station, tiny bottles of Victorian pharmaceuticals (the only one still full was Oleum Lavandin, which smelt as good as new), a wages book from a hairdresser in the 30's, and a whole stack of newspapers from the late 19th - early 20th century. And soot. So much soot.