YAYOG a book called You Are Your Own Gym https://www.amazon.co.uk/You-Are-Your-Own-Gym/dp/0091955424
The is a book describes itself as
the bible of bodyweight exercises
and also states
use the body you have to build the body you want
The book describes 125 different exercises in which you use your own bodyweight for resistance (instead of using weights or exercise machines).
Does it work? In short - YES. If you follow the program you will notice a difference in muscle tone within weeks!
Note that the books tells you how to make the exercises easier or harder.
E.g. push ups - easier: raise hands off floor, harder: raise feet off floor.
There are 4 levels to the program: Basic, First, Master and Chief.
Each level has a 10 week cycle of exercises. The cycle ensures that no 2 adjacent days exercise the same muscle group.
The program was developed by a former US Marine (Mark Lauren) who used to train US Special Forces - so don't expect an easy ride.
The program is not as frenetic as some you might see on TV. Good slow form is preferred over rapid actions.
I have been doing it on/off for the last 12 years (since I turned 50).
I have modified the program to 4 x 15 minute sessions a week - so about an hour a week.
I do a 2 minute warm up. Then I do the exercises for the day. When finished I do 2 minutes of stretching.