The starting point for Bitcoin is the Whitepaper which was issued on 31 October 2008, it is only 9 pages long and can be found here. The paper was issued by a person or persons unknown going by the pseudonym of Satoshi Nakamoto, it is a technical work of genius. You can buy the T-shirt here (but only after you have read the paper!).
There are now many books seeking to explain Bitcoin, the two I recommend are Digital Gold, which is the backstory and history of the early days of Bitcoin, and The Bitcoin Standard, which is a very easy to read explanation of the technology of Bitcoin and how it sits within the history of money.
If you do not have time to read these books you have two other choices. I set out here a 4 minute read on ‘What Bitcoin is?’. Alternatively you could watch a 7 minute video here.
Good luck and enjoy!