One of the classic Systematic Theology books is Herman Bavinck's Reformed Dogmatics. If you are looking for a great price (40% off) on a classic book click here.
Check out Richard Gaffin's endorsement of this book:
"Arguably the most important systematic theology ever produced in the Reformed tradition - I have found it to be the most valuable - English-speaking theology throughout the 20th century until now has been singularly impoverished by not having at its disposal a translation of Bavinck’s Dogmatiek in its entirety. The appearance of this volume, with the remaining three planned to follow in the near future, will be an incomparable boon for generations of students, pastors, teachers and others, serving to deepen understanding and enrich reflection in both historical and systematic theology."