Both the Simple and Complex* Google Book APIs let you request essentially a small / medium / large version of the cover in the request itself.
I also just kind of want to weigh in with others and recommend finding an API that lets you request different image sizes or otherwise consider redesigning where these book covers appear if fitting that design means the image is going to get stretched out of proportion or look pixelated. Layout ultimately takes second-seat to the quality of the content itself, and users ultimately don't care about your design unless it's hindering their experience, then you'll hear a-plenty.
* I don't really know what the non-simple version is called officially, so hey - "complex" :)
// Michael Schofield
The web for libraries your thing? I write about it here at www.ns4lib.com
From: Code for Libraries [mailto:[log in to unmask]] On Behalf Of Keith Jenkins
Sent: Wednesday, December 04, 2013 1:16 PM
To: [log in to unmask]
Subject: Re: [CODE4LIB] book cover api
I was a bit surprised that these techniques from 2005 still work...