Reality is Not What It Seems. The Journey to Quantum Gravity.
by Carlo Rovelli. The author is a theoretical physicist and leader in the quantum-loop gravity world. Quantum-loop gravity attempts to fit gravity into a similar framework as quantum field theory: gravity field-lines "loop" much like magnetic field lines is the simple physics experiment with magnets and iron filings. Quantum gravity postulates that both space and time are quantized at Planck levels (defined by units and Planck's constant). String-theory is the other competing theory in theoretical physics attempting to pull together quantum mechanics, relativity, and gravity into a consistent physical explanation.
The book is successful in giving a popular account of science at it' highest levels. It is a little short on philosophical consistency, however.