Ringworld by Larry Niven (1970)
The ringworld. It has inspired stories and games like Halo, and remains one of the biggest BDOs conceived. Six hundred million miles long and ninety million miles in radius.
Yes, this book was written in 1970, but I still think it holds up today… mostly. It does have some odd 70’s attitudes, and the characters are a bit wooden, but just the concept of the ringworld and the technical details the book delves into makes it a fascinating read.