Rutherford (1998) is a detailed scholarly study of Leibniz's Theodicy(d).
Frederick C. Beiser(d), German Idealism: The Struggle Against Subjectivism, 1781-1801, Harvard University Press, 2002, part I.
Frederick C. Beiser(d), German Idealism: The Struggle Against Subjectivism, 1781-1801, Harvard University Press, 2002, p. viii: "the young romantics—Hölderlin, Schlegel, Novalis—[were] crucial figures in the development of German idealism."
Beiser(d) reviews the commonly held position that Schopenhauer was a transcendental idealist and he rejects it: "Though it is deeply heretical from the standpoint of transcendental idealism, Schopenhauer's objective standpoint involves a form of transcendental realism, i.e. the assumption of the independent reality of the world of experience." (Beiser, Frederick C., Weltschmerz: Pessimism in German Philosophy, 1860–1900, Oxford: Oxford University Press, 2016, p. 40).
Beiser, Frederick C.(d), Weltschmerz: Pessimism in German Philosophy, 1860–1900, Oxford: Oxford University Press, 2016, p. 213 n. 30.