euler - introductio in analysisin infinitorum -- brilliant work of euler from 1748 containing many striking results. english translation available.
bernhard riemann - on the number of primes less than a given magnitude -- riemann's one paper (~15 pages) on number theory, which introduced his famous zeta function (english version available in riemann's zeta function by edwards, a book dedicated to the very rich subtext of this terse paper)
shannon - a mathematical theory of communication -- seminal paper founding information theory
schrodinger -- find yourself a decent exposition of the analysis of the hydrogen atom using schrodinger wave mechanics. learn where all that junk they taught you in high school chemistry actually comes from!
Feynman Lectures on Physics -- comprehensive account from the man who knew physics as well as anyone.
ahlfors - complex analysis -- best text i know of on this subject in mathematics that shows up in the most surprising places in the sciences.
landau & lifschitz - course on theoretical physics -- 10 volumes on modern physics from classical mechanics to electrodynamics, relativity, quantum mechanics, thermodynamics, fluids, etc. from nobel prize winner lev landau.
Fourier Analysis - t w korner -- intro to fourier analysis with many applications (after all, applications are the whole point of fourier analysis) from your basic heat equation stuff to calculating the age of the earth and other interesting things.
i think that in compiling this list, you will find two things to be true:
1. increasingly (in the last century, for example), important work is not (initially) published in books, but in papers.
2. trying to read the original works is fun for about 5 minutes. if you really want to learn, modern expositions in textbooks tend to be far better than the originals.