Someone who owned 10,000 hardcover books and the same number of vinyl records could bequeath them to descendants, but legal experts say passing on iTunes and Kindle libraries would be much more complicated. And one’s heirs stand to lose huge sums of money. “I find it hard to imagine a situation where a family would be OK with losing a collection of 10,000 books and songs,” says Evan Carroll, co-author of “Your Digital Afterlife.” “Legally dividing one account among several heirs would also be extremely difficult.” Part of the problem is that with digital content, one doesn’t have the same...