Number Theory: An approach through history from Hammurapi to Legendre by Andre Weil is a classic. In it Weil covers some thirty-six centuries of arithmetical progress, with a close account of the founding fathers of modern number theory: Fermat, Euler, Lagrange and Legendre.
The Shaping of Arithmetic after C.F. Gauss's Disquisitiones Arithmeticae
edited by Catherine Goldstein, Norbert Schappacher, Joachim Schwermer. In this book eighteen authors - mathematicians, historians, philosophers - discuss the impact C.F. Gauss's Disquisitiones Arithmeticae (1801) has had on mathematics.
From Kant to Hilbert: A Source Book in the Foundations of Mathematics Volume I and Volume II
by William Bragg Ewald. An historical overview starting from Kant's Critique of Pure Reason, widely taken to be the starting point of the modern mathematics, up to the end of the nineteenth century with Hilbert. Ewald's two-volumes contain translations of works by Bolzano, Cantor, Dedekind, Gauss, Hamilton, Kronecker, Riemann, Poincare, and Zermelo showing the links between algebra, geometry, number theory, analysis, logic, and set theory.
Geometry by Its History by Alexander Ostermann and Gerhard Wanner. This is an undergraduate book, but that's not the point: it's one of the most beautiful historical books on geometry out there.
The Emergence of the American Mathematical Research Community, 1876-1900: J. J. Sylvester, Felix Klein, and E. H. Moore by Karen Hunger Parshall and David E. Rowe. In this book we see what the title suggests: Sylvester at John Hopkins, Moore at the University of Chicago, with Klein popping over to tour the mathematical scene.
The Mathematics of Plato's Academy: A New Reconstruction by David H. Fowler. In this book Fowler examines what we really know of the mathematics done in Plato's Academy, and before Euclid: not much! A brilliant book written by a classicist for fans of Euclid.
Get well soon.