Here is an incomplete list of pre-1900 books that I read, enjoyed and strongly recommend (I apologize for some repetitions):
Collected works of Archimedes.
Ptolemy, Almagest (yes, this is a math book:-)
Kepler, Stereometry of wine barrels.
Newton's Principia,
Complete works of Abel and Riemann, Laguerre and Stieltjes.
Gauss, General investigation of curved surfaces (available in English)
Fourier, Analytic theory of heat.
Fourier, Analyse des equations determinees (this is a rare book. Available on my web page).
Complete works of Chebyshev (available in Russian and French)
Maxwell, Treatease on Electricity and Magnetism. (There is a nice paper of F. Dyson, Missed opportunities, where he explains how much Mathematics would gain if mathematicians read this book. I completely agree with Dyson).
Painleve, Lecons, sur la theorie analytique des equations differentielles, professees a Stockholm, 1897.
Picard and Poincare, of course...
BTW, I disagree with designation "pre-modern" for the period before 1900. From my point of view, "modern period" begins with Abel. There is no substantial difference between Laguerre or Stiletjes and XX century mathematics.