Microcosmic God By Theodore Sturgeon
Mimsy Were The Borogoves By Lewis Padgett (alias of Henry Kuttner and his wife, C. L Moore)
The Little Black Bag by C M. Kornbluth
Dark They Were, and Golden-eyed by Ray Bradbury
The Great Work of Time by John Crowley
Or All the Seas with Oysters by Avram Davidson
Spacetime for Springers by Fritz Leiber (not really SF but highly recommended, esp if you have cats)
The Darfsteller by Walter M. Miller
All Seated on the Ground by Connie Willis (Christmas SF)
For more recommendations check out this thread:
https://www.mobileread.com/forums/showthread.php?t=43439