According to the entry in Lewis and Short, the accusative often refers to what was specifically written as "a line" or "a written composition, writing, treatise, book, work, etc."
In all the examples they give, the prepositional phrase, de + abl., refers to the subject matter of what was written. Often the accusative is used together with the ablative phrase, referring both to the material and the subject matter of the material:
quoniam de re publicā [of the republic] multa quaesierint et scripserint, Cic. Rep. 1,
librum de rebus rusticis [of rural matters], Cic. Sen. 15, 54
scripsi etiam versibus tres libros de temporibus meis [of my times], Cic. Fam. 1, 9,
in Catone Majore, qui est scriptus ad te de senectute [of old age], Cic. Lael. 1, 4
scriberem ad te de hoc [of this] plura, si Romae esses, Cic. Att. 6, 4, 11
Hermae tui Pentelici, de quibus [of which things] ad me scripsisti, Cis. Att. 1, 8, 2; 1, 9, 2 et saep.
Although the forgoing resembles the English usage, the accusative is also used in senses that are not quite consistent with English. A notable example of this is "forman et situm agri alicui scribere," which in English would be better translated with the word "describe":
to describe to anyone the form and situation of a farm.
Lewis and Short refer to this sort of usage as "the accessory idea of intellectual action, of written composition of every kind, to write, write down, compose, describe, depict; to draw up, communicate, announce in writing."