I have no idea what percentage. Things to take into account is that they are relatively recent discovered astrophysical phenomena that are currently under investigation. I would not expect many books to discuss Fermi bubbles.
Or is you question about suggestions for reviews of the topic including monographs and textbooks? (Honestly, this is a bit outside my area so I cannot really make any informed suggestions here.)
In addition to what Andrew said, it may also be the wrong question to ask. Books with titles like "Gravity" or "Gravitation" usually focus on a theoretical description of gravitation in terms of the general theory of relativity, which on galactic scales is not really practical.
Fermi-bubbles appear to be a fairly recent astronomical observation, which doesn't seem to be understood in terms of models or theory at all yet. So most likely zero books on gravity discuss that. If at all, then books on astronomy would discuss it. But since it is so recent probably you would have to go to journals and not books.
Gernot Heißel when it comes to forecasting space radiation, I'd argue that Fermi Bubbles or Sunspots are the most important 2-D general relativity solutions
Eric Lee D'Aleo if you wrote a book then you did a massive literature review first, so you are probably the best person to answer your own question!
And I really do echo what Gernot said. A book on astrophysical phenomena rather than gravity is more likely to say something even if general relativity is vital in the description.