The websites below have many useful resources for further study.
Desiring God (John Piper) – http://www.desiringgod.org/
Ligonier Ministries (R. C. Sproul) – http://www.ligonier.org/
Grace to You (John MacArthur) – http://www.gty.org/
Anchored in Truth (Jeff Noblit) – http://www.anchoredintruth.org/
Founders.org – http://www.founders.org/
Highlands Ministries (R. C. Sproul, Jr) – http://www.highlandsministriesonline.org
ESV Online Bible
Logos Online Bible
Christian Classics Ethereal Library – http://www.ccel.org/
This resource contains many writings (click browse on the menu) by various teachers in the history of the Church. There are several reformed teachers including John Calvin, Jonathan Edwards, Matthew Henry, and Charles Spurgeon.
Structured Study Resources
Ligonier Connect http://connect.ligonier.org/catalog
Additional Study Resources