Read the excerpt from A Black Hole is NOT a Hole. It's terrific fun to read what the original thinkers had to say for themselves. I read parts of Relativity: The Special and General Theory by Albert Einstein, and found the letter about Einstein forgetting to eat lunch in Einstein: His Life and Universe by Walter Isaacson.

I also reviewed parts of Isaac Newton's Principia: The Mathematical Principles of Natural Philosophy (translated by Andrew Motte), to see what Newton himself had to say.

What makes these books credible sources?

The books give opinions and arguments about science.
Einstein and Newton are experts in their fields.
The books describe Einstein’s and Newton’s inner feelings.
Einstein and Newton persuade readers to their opinions.