Coming into the Vernon East/Takoma Park metro from Vernon Square East, you'll find one just a few feet away in a ticket booth.
Notes[]
You can actually get a maximum of 25 Speech skill books when you consider the following:
You can only get either the Lying, Congressional Style book in your Megaton House or in your Tenpenny Tower Suite, not both. This subtracts one book.
The skill book in Allistair Tenpenny's suite respawns if you let the ghouls into the tower. This adds one book.
The Paradise Lost book is also a Speech skill book. This adds one book.
Bugs[]
If you buy the Love Machine Theme, take the skill book and then go buy a misc item for your house (Pristine Nuka-Cola Machine, My First Infirmary) you will get another skill book. Rinse and repeat to get a total of 6 skill books as long as you can spare the caps.
It's possible for the book in your Tenpenny Tower Suite to respawn. To have this happen, get the book while the tower is still owned by Allistair Tenpenny and his residents. If you then allow the ghouls to take over the tower, Tenpenny Tower is replaced with "Ruined Tenpenny Tower" and the book will re-appear.