The following books are collections of sermons and essays unknown to many in the Evangelical and Reformed worlds. They can be found in PDF files for FREE at http://books.google.com/:
(1) Sermons and Essays of John MacLaurin (or Mc’Laurin, or even M’Laurin). John MacLaurin was a Scottish Presbyterian Minister of the early and mid 18th Century. He was a correspondant of Jonathan Edwards. His books are very hard to find. This makes Google Books PDF files such an amazing resource. The link to the three sermons and essays is here and the link to eight sermons is here. You can find the works of MacLaurin and the sermons of MacLaurin there. Some of the sermons I would recommend would be “Glorying in the Cross,” “God’s Cheif Mercy,” and “The Knowledge of Christ Crucified the Sum and Substance of of Saving Knowledge.” As far as his essays go, the first one I would recommend to you is his treatise on Grace. It is probably the best treatment of the subject of grace I have read. Its not the easiest reading but it is well worth the effort.
(2) A Series of Sermons on the Most Important Principles of Our Holy Religion by Alexander McWhorter. McWhorter was one of the founders of Princeton Seminary. He was an amazing Presbyterian Preacher. His sermons are very hard to come by but well worth having. The link to the PDF files on Google books is here