Pretty much essential if you want to give your players the full swath of the US Government for options.
This is close as you can come to an "Expansion pack" to the Agent's Handbook and is something you can hand your players.
$10 feels like the perfect price for something like this honestly.
Have a player who wants to play an armed park ranger? With this you can! It even has an incredibly helpful page showing what pages all of the agencies are between this and the Agent's Handbook.