The regular course of studies, the years of academical and professional education have not yielded me better facts than some ________ books under the bench at the Latin School.

Than in the sentence sets up the contrast between what precedes it and what follows it. The author expresses his discontentment for the regular course of his studies in school i.e. the academic and professional education of school and college have not yielded much knowledge to him but some unimportant books have fulfilled his desire for knowledge. Hence, “idle” in (2) fits our prediction and is the answer.