I love relativity. I am super interested in the implications it has on the most fundamental element of the very fabric of reality, time. The way relativity predicts time dilation, both special and general relativistic, fascinates me. The special theory of relativity has been around for over 100 years now, being announced in 1905 by Albert Einstein. The general theory of relativity was announced ten years later, after a break to work on quantum mechanics I might add, on November 25 1915 by Albert Einstein. Since then the scientific communities of theoretical, experimental, and observational physicists have come a long way. Now we have fields like computational relativity, where physicists use numerical methods to simulate relativistic effects, mathematical relativity, where physicists and mathematicians are working on improving mathematical methods related to relativity, and with the direct discovery of gravitational waves in September 2015, 100 years after the invention of the theory of general relativity, we have observational relativity, and more.
There are numerous texts on special relativity out there, but the one I recommend is Spacetime Physics by Taylor/Wheeler published in 1966 by W.H. Freeman & Company.
There are also numerous texts on general relativity of varying levels, but I have a few that I like.
In the spring of 2016 in Astronomy 320: Cosmology at Western Washington University, I presented a research paper (not my own research, but research done by others that I gathered and presented) on Measuring Galaxy Cluster Masses with Gravitational Lensing. Now I have provided it here for you.
There are even more resources from which to learn on the internet. Here I give a few of them.
PBS Spacetime is a great channel on YouTube that delivers high quality videos that are accessible to almost anybody by not using super math-y terms and presenting their information in a fun and casual way (plus a lot of pictures).
This video is the first in a series of lectures uploaded to the internet for anyone to see from Stanford. They are of course general relativity in its full form, so they're not for the faint of heart.
Over the past few years I have collected quite a number of research papers on relativity, mostly general relativity, and now I have put them here for you to browse.
Note: Because these are research papers developed by people with their Ph.D.'s in this field, these papers are graduate/post-graduate level, but they are nevertheless an interesting resource.