I would have thought my rarest, most valuable books were my four-volume first edition of the Idylls of the King, illustrated by Gustave Dore (1859).
They're lovely: folio-sized, one bound in morroco, gilt spines, embossed covers, tissue paper protecting each engraving.
I own some nineteenth-century dance manuals, a school reader from 1847, a few soldiers' reminiscences published just after the Civil War, a first edition Joyce Carol Oates complete with author signature and slightly tattered dust jacket, and a Margaret Atwood (ditto).
None are as valuable as this book I picked up in my LYS a few weeks ago:
It's now retailing on Amazon for around $500. I've no idea why.
Oh never mind. That was for the hard cover edition. I guess I can't retire as a rich bibliophile after all.