In The Ultimate RPG Character Backstory Guide, James shares 100 different exercises to let you develop your own characters like he does for his podcast. Some are more thought-provoking than others, while some are highly entertaining, but all of them have value and could contribute to better fleshed-out characters and more dynamic adventuring groups.
While the exercises within are intended to be used with any RPG system you like, most of my pen & paper gaming experience is with various versions of Dungeons & Dragons, so I am primarily evaluating the book from that perspective; your mileage may vary. That having been said, the only issue I have with the book is the name, itself. The activities contained within this book are all interesting and useful, but in the strictest sense of creating "backstory" for a D&D character, they don't all necessarily apply - at least in most of the games I have played. This is primarily because creating a player character's background is part of the character creation process, which typically (and in all of the games I've been a part of) occurs completely before the player characters meet. So, any interactive exercises between player characters, while really cool and certainly contributing to a better team/party cohesion... wouldn't have been in the "backstory" section of the character, but would be played out during gameplay. Again, I have no problem with the activities, but to me, this is not 100 exercises to develop your backstory, but would definitely be 100 exercises to develop your character... just some take place after the "backstory" phase.
Now, one exception would be if you're putting together a party that's high level and you don't want to have them all meet at an inn (at level 15, let's say) and head off as a new party, then you could use some of these collaborative exercises sort of between character creation and actual gameplay to create some "party backstory" that would provide some highlights of major events that occurred in their past, and contributed to the party dynamic. Perhaps a couple of the characters have known each other since childhood and they have some shared backstory that affects their relationship. Maybe two others met in military service and they have a shared interaction from back then. And maybe, just maybe, even though you're starting the game at level 15, you decide that the current party has been adventuring together since level 10 and you use a few of these activities to create some shared history, anecdotes, secrets or inside jokes to provide a richer team dynamic. (As a matter of fact, as a fan of the role-playing aspect of RPGs, that sounds absolutely fantastic and I highly recommend doing exactly that if you're about to launch a mid to high level campaign.)
The second exception is if you are creating backstory with characters that aren't in your current party, but some party that you were a part of in earlier levels. This would be backstory, in the truest sense and, while the resulting depth of relationships don't apply directly to the current party members, these earlier experiences could definitely shape your character's perception of his world, in general, which could color how he approaches his new party. Did a double cross in his past cause him to distrust rogues? Did an unrequited love leave him tongue-tied around female Elves? Did his previous party fully accept and support his outlandish manner of celebrating his cultural heritage by participating in some obscure holiday from his homeland and now he can't fathom that anyone wouldn't want to participate? These could quite potentially also serve as excellent hooks for the game master / dungeon master to use in later quests. (Your rogue isn't into charity and isn't interested in accepting a quest with such a paltry reward, but wait... did they say it was for a blonde Elvish fighter named "Elesandra"... surely not the same Elesandra?)
The third exception in which all of these are perfectly fine to be considered as backstory is... if you're not developing player characters to play D&D. If you're a GM/DM creating non-player characters or an author writing a story, these exercises could all be put to good use to create more intricate, detailed, believable and, ultimately, much more interesting characters to populate whatever literary world you're building.
For those who are concerned about the fact that some of the activities are not strictly before forming a party, I have gone through the exercises and conducted a quick count of how many fit this strict definition of "backstory" and about 87 of them did not rely on current party members. A couple of these 87 include options that might call for "magical enforcement" or a curse on the character that would require GM/DM buy-in if it is to actually have an effect in-game that isn't just the character's own emotions or actions, but that's only a couple or so.
The book is divided into three sections, Humble Beginnings for low levels (levels 1-7), Veteran Heroes for mid levels (levels 8-14), and Myths and Legends for high levels characters (levels 15-20). Most of the activities that involve current party members are in the Veteran Heroes section, as Humble Beginnings is more about, well, actual before-becoming-an-adventurer style backstory and Myths and Legends is more about fleshing out larger-than-life details and quirks about your character. And, again, even the activities that ask for the name of a party member could be backstory if your character isn't just starting out and you use some imagined previous party members to do the exercise.
If you're a fan of deeply developed characters and want new ideas on how to breathe life into your characters (PC or NPC)... or to develop characters for any creative work that don't have a one-dimensional feel to them, The Ultimate RPG Character Backstory Guide should have a few tricks up its leaves for you.