"Bumps and Wiggles: An Introduction to General Relativity" by Gary Felder

a very good introduction to fundamentals of general relativity in a very simple way. it is a good start
for general relativity theory.

"RELATIVITY special and general theory" by Albert Einstein
for the preface and other matters use the upper link.
chapter 18 "Special and General Principle of Relativity"
chapter 19 "The Gravitational Field"
chapter 20 "The Equality of Inertial and Gravitational Mass as an Argument for the General Postulate of Relativity"
chapter 21 "In What Respects Are the Foundations of Classical Mechanics and of the Special Theory of Relativity Unsatisfactory?"
chapter 22 "A Few Inferences from the General Theory of Relativity"
chapter 23 "Behavior of Clocks and Measuring Rods on a Rotating Body of Reference"
chapter 24 "Euclidean and Non-Euclidean Continuum"
chapter 25 "Gaussian Co-ordinates"
chapter 26 "The Space-Time Continuum of the Special Theory of Relativity Considered as an Euclidean Continuum"
chapter 27 "The Space-Time Continuum of the General Theory of Relativity Is not an Euclidean Continuum"
chapter 28 "Exact Formulation of the General Principle of Relativity"
chapter 29 "The Solution of the Problem of Gravitation on the Basis of the General Principle of Relativity"

chapter 30 "Cosmological Difficulties of Newton’s Theory"
chapter 31 " The Possibility of a “Finite” and Yet “Unbounded” Universe "
chapter 32 "The Structure of Space According to the General Theory of Relativity"

appendix 3 "The Experimental Confirmation of the General Theory of Relativity"

Bibliography

 

RELATIVITY general revision

general review on theoretical bases of relativity. here you can find some pictures where
each picture is a link to certain topic in relativity

until now all above are very important to basics but the following are extra topics with good
importance

 

"General relativity II: a deeper look" by Gary Felder

it is deeper look to general relativity containing some mathematics which may be difficult.