I finally bought the 500 Year Diary.
For those who are not aware, the 500 Year Diary is a book that was carried around by second Doctor, portrayed by Patrick Troughton. It was a nonverbal way of communicating how old the Doctor was, to viewers. It helped to add to his alien nature, by showing that he lives so much longer than humans. At the time of the second Doctor, it has not yet been established that he had two hearts (that would first be mentioned in the third Doctor serial, Spearhead from Space.) His people, and their planet, Gallifrey, had not yet been named (it was first shown in the final second Doctor serial The War Games and named in The Time Warrior.) Other iterations of this book would be seen later, such as the Doctor Who TV movie (900 Year Diary,) a mini episode titled History of the Doctor (Twelve Hundred Year Diary,) and the twelfth Doctor adventure The Girl Who Died (Two Thousand Year Diary.)
Many people have made their own, much nicer version of the 500 Year Diary, as a cosplay prop or to have for their own use. I just payed the $7 online to finally get a copy that BBC licensed and Bif Bang Pow! printed. Along with the River Song printed journal it’s nice to have around the house. It’s also nice that the pages inside are unlined and printed to look older. If you’re not the crafty sort it’s fun to put a few dollars into having around the house.