I love the design of these Halve Bookends which work as either fish bowls or vases. With fish in them it sort of looks like your fish have a massive aquatic world hidden inside your tomes of knowledge (or trashy beach reading, whatever you have on your shelves). Or you could just put flowers in and use them as vase bookends.
Or maybe go a little crazy and have flowers on one side and fish on the other. Or marbles. Marbles would be nuts. Or actual nuts. Peanuts. Squirrels on one side and nuts on the other. Endless possibilities. $49 for the paid at Gnr8, ships free.
Innovera Nonskid Heavy Gauge Steel Universal Economy Bookends, 4 3/4 x 5 1/4 x 5-Inches, Black, 2 Bookends (54055)
Universal Economy Bookends, Standard, 4 3/4 x 5 1/4 x 5, Heavy Gauge Steel, Black, 1 Pair (UNV54051)