Read old articles instead books.

Solution 1:

Many of the old journals are available online, though some may require subscription. For example, Crelle's Journal, going back to 1826, is freely available at http://gdz.sub.uni-goettingen.de/no_cache/dms/load/toc/?IDDOC=238618

Or you could consult the works of Gauss

http://gdz.sub.uni-goettingen.de/dms/load/toc/?PPN=PPN235957348

or Euler

http://www.eulerarchive.org/

Solution 2:

Stephen Hawking's God Created the Integers: The Mathematical Breakthroughs That Changed Historycontain many old articles. (as Bruno mentioned already)

Timothy Gowers' The Princeton Companion to Mathematics contain concepts in article form by contemporary mathematicians.

Stewart Shapiro's The Oxford Handbook of Philosophy of Mathematics and Logic contains many papers on logical concepts, again, by contemporary authors.

Blackwell guides can be useful such as The Blackwell Guide to Philosophical Logic (Blackwell Philosophy Guides).

As for online versions please see the related link: List of Interesting Math Blogs

And you are perhaps already familiar with Stanford Encyclopedia of Philosophy; for mathematical biography Mactutor History of Mathematics is resourceful.

Finally, I personally visit Interactive Mathematics Miscellany and Puzzles to understand basic concepts.

Solution 3:

I haven't browsed their collection in detail, but Numdam has a lot of good stuff.

Solution 4:

Many authors have pre-publication versions of articles they wrote on their personal web page and sometimes old articles are now in the public domain. You can often locate these articles using either: http://scholar.google.com/ or http://www.scirus.com/srsapp/ by searching using some appropriate string or using the author's name.

Solution 5:

http://www.archive.org/ has lots of old stuff.