Someone asked me about middle knowledge after I made mention of the term in a recent post at The Preacher’s Library. I say on page 389 of my book The Other Side of Calvinism that middle knowledge is “knowledge of what will or could or would happen.” This is also called Molinism, after Luis de Molina (1535-1600), who taught that through God’s middle knowledge, “in virtue of the most profound and inscrutable comprehension of each faculty of free choice, He saw in His own essence what each such faculty would do with its innate freedom were it to be placed in this or in that or, indeed, in infinitely many orders of things—even though it would really be able, if its so willed, to do the opposite.”
I was introduced to the term via William Lane Craig’s The Only Wise God: The Compatibility of Divine Foreknowledge and Human Freedom (Baker, 1987). The more recent and notable books on the subject of middle knowledge are, from a philosophical point of view, Molinism: The Contemporary Debate, edited by Ken Perszyk (Oxford, 2011). From the religious perspective there is Middle Knowledge: Human Freedom in Divine 
Sovereignty, by John D. Laing (Kregel, 2018). I recommend Luis de Molina: The Life and Theology of the Founder of Middle Knowledge, by Kirk R. MacGregor (Zondervan, 2018), and William Lane Craig’s contribution to Divine Foreknowledge: Four Views, edited by James Beilby and Paul R. Eddy (IVP Academic, 2001).