I highly recommend the Reformed Books Online website.
Our purpose is to promote historic, reformed Christianity by providing in one location a collection of the best theological literature from 1800 to today available for free on the internet. Select works from the reformation and puritan periods are included as well.
The technological advance of online libraries (Google Books, Internet Archive, etc) has opened up opportunities never seen before. We are culling the cream of the crop for you so that you can enjoy the best Christian literature from around the world that has previously been unavailable.
This site is intended to be popular enough for the new Christian and at the same time in-depth enough to be helpful to scholars.
Our focus is on the time period from 1800 to the early 1900’s, which this site is intended to be a library for. We are providing exhaustive lists of available online works for these 1800 to early 1900’s authors. That which is included from the reformation, puritan and contemporary periods is much more selective.
The reason for the (not always followed) AD 1800 cut off point is that Post Reformation Digital Library is an exhaustive, scholarly online collection of reformed works from the 1500’s to 1800. Their site is highly recommended. The main reason for the focus on works before the early 1900’s is that is when modern copyright laws put an end to all books being in the public domain.
See here to Get the Most out of our Site.