Diy shed floor advice?

I would build up 3, 24' beams using double 2x8's. 1 on each 24' side and one parallel down the center. You could have 2 rows of 2x6 joists between them. 8' spaced piers is a bit too much unless you let your wall sheathing drop down and cover the perimeter beams to help hold them up. For a shed you don't want 8' walls anyway. It'd look way better to incorporate the floor thickness into the 8' sheathing height anyway. You'd have 84" ish studs instead of 93's. Believe me, this will look less Snuffy Smithish. If I were building that shed I might even consider setting 6x6 posts @ 6' centers for the foundation and notching them to support and and windhold the floor system instead of laying it atop of masonry pillars.

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it should be enough to support your weight the books will probably weigh more than anything if the book shelf has any size to it and your floor shouldn't sag most homes are 4 foot between piers so for a shed 8 would be fine as it has not even close to the weight a home has

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Slightly overkill

Single 2x8 x 16' beams and 24" centers would suffice..

You'll be fine. Only concern would be the footings. Depending on where you live, 2 feet may not be deep enough. Consider frost heave.