A quote from Philip Graham Ryken in his book titled Thomas Boston as Preacher of the Fourfold State (Rutherford Studies in Historical Theology) pertaining to the law and gospel distinction in Thomas Boston:
It is not the purpose of the present thesis to minimize the importance of Thomas Boston’s federalism or to re-evaluate his place within the history of covenant theology. There are important parallels between his sermons on the covenants and those on the fourfold state. Indeed, Boston’s introduction to A View of the Covenant of Grace echoes the dichotomy between sin and grace which lies at the heart of the Fourfold State:
‘My design is, under the divine conduct, to open up unto you the two covenant of works and grace; and that because in the knowledge and right application of them the work of our salvation lies; the first covenant shewing us our lost state, and the second holding forth the remedy in Jesus Christ; the two things which, for the salvation of souls, I have always thought it necessary chiefly to inculcate. (XI, 178; cf. I, 279; VIII, 115; XI, 252). (85)
Also related, over at the Reformed Reader blog, Rev. Shane quotes Bavinck in regards to the sharp law and gospel distinction here.