For me Migne's Patrologia Latina and Graeca. I've had online access to the Latin portion for a decade and the Greek is now available online too, but I'm no closer to reading ancient languages.
I've always been intrigued by it though as I've read in multiple places that only a small portion of its material has ever been translated into English.
Another candidate would be the Suda
The oldest historical text I have that's relevant to my interest is How The Jap Army Fights, published in 1942. That it was my maternal grandfather's copy, and he passed away in 1945, some 23 years before I was born, makes it awfully special.
The more obscure title is probably The Last Flight of Ensign C Markland Kelly, Junior, USNR, Battle of Midway, June 4 1942, by Bowen Weisheit.
Years after his loss, Ensign Kelly's father asked his friend Weisheit, a retired Major in the Marines, to find out exactly what happened to his son, a Wildcat pilot flying off the Hornet. Even those who are familiar with the Battle of Midway don't know about the so-called "Flight To Nowhere," but Weisheit was one of the first to truly uncover what probably happened.
There are some problems with the text, not least of which was the author's unwillingness to correct minor errors, even when Midway experts pointed them out. The other big problem is that it's very difficult to obtain copies... it was privately published, and with the passing of the author some years ago, it's unlikely it'll ever be made available again. I was lucky enough to "know a guy" at the right time.