The OED says “A shortening or modification of ENCYCLOPÆDIA (itself due to an erroneous Greek reading), perh. intended to convey more obviously the ostensible sense ‘circle of learning’.” First cite for the ‘circle of learning’ sense:
1636 SIR H. BLOUNT Voy. Levant 85 This Cyclopædia hath beene observed to runne from East to West: Thus have most Civilities, and Sciences come..from the Indian Gymnosophists into Egypt, from thence into Greece, so into Italy.
And for the ‘book’ sense:
1728 CHAMBERS (title), Cyclopædia, or General Dictionary of Arts and Sciences.
(Aldi, you’ve got to start making more comments, so you can break the 31-comment barrier and get your second box!)