The Summer Hikaru Died, Vol. 4 von Abigail Blackman

"While 'Hikaru' is not the Hikaru he once knew, Yoshiki must admit that this new entity has become a friend in his own right. But his true identity remains a mystery...even to the impostor himself! When their research into the town's history brings them to the library, the boys discover an unsettling clue about the mysterious happenings in the region. Though, the question still stands of what 'Hikaru' and the Nounuki-sama mentioned in Hikaru's note might be-and whether they aren't in fact one and the same...--