This is pretty much exactly what the titles says it is; a list of 100 system agnostic ideas for why or how someone became a ghost. Between this and 100 Signs of Ghosts, I think 100 Ways to Become a Ghost is slightly better in execution as more of the list seems usable to me. Some of the entries for 100 Signs of Ghosts are either too vague to be helpful (such as #97 Warning), or seem like they be more appropriate as a the basis for a supernatural encounter rather than as the sign of ghosts being nearby (such as #27 Flying banshee).
The other plus about 100 Ways to Become a Ghost is that some of the entries can help define your setting, or bring a little bit of background into the game with them. For example, #39 Forgotten by descendants, implies the culture the ghost came from practices some form or ancestor worship. Why else would a neglected ancestor become a ghost? That entry can help shape the culture, country, or even entire world of your game setting. Do all the people believe in ancestor worship? What kind of rituals or celebrations do they have? How do they feel about Clerics and Raise Dead spells? So in addition to giving GMs a quick way to generate an interesting origin for the ghost the PCs just encountered, it also gives them seeds for setting ideas.
Plus, you really can't beat the $1 price tag. :)