Unless it's an old clunker, I've always looked for proof in addition to the service book, and thus far it's resulted in cars that have additional receipts and invoices that I buy, as alluded to by eatcustard, it's all too easy to have a book full of fake stamps, easy enough to check on a newer car but harder the older the vehicle.
My old 528i had arch files full of every service and the parts used, as does my 530i, rarer to find cars with such detail but IMO it indicates previous owners who cared or thought more of ownership of the car in question.
The condition should match the age and mileage, ive seen far too many cars with a stamped book FSH that clearly don't look well looked after to make me wary of just a stamped book.