It's a blog! Mainly of book reviews.
How to deal with p16:
Wait over a year then read a different book on General Relativity.
Start this book over from the beginning.
Get completely stuck at p16 again.
Desperately look on the web.
Discover you need calculus of variations.
Look it up.
Find Dirac used an archaic notation.
Learn weird notation.
Realise you didn't follow the bit you thought you understood.
Finally comprehend what was intuitively obvious anyway!