I'm looking for some kind of plugin and/or widget to display a list of book covers in the sidebar. Are there any good options out there?
There are quite a few plugins tagged
books in official repository.
A lot of them seems to pull covers from services like Amazon, which may or may not fit your needs. If you want to simply display some self-hosted images with links and basic categorizing/sorting, personally I like to use Links for such.
I use GoodReads.com, in which you can create shelves of books you've read or want to read or unlimited other categories. Once you have a shelf with books on it, you can create a widget to show those books on your website.
Here's how: Once you've created a goodreads account, go here: http://www.goodreads.com/user/edit and click the "Widgets" tab. They have a handful of different styles to choose from, most of which can be customized further, and then it provides the code to represent that widget. Most of the widgets are (or can be) dynamic, so once you've created the widget, if you change the books on the shelf it represents in goodreads.com, then the widget will reflect the change.
Then, I added a Text widget to my wordpress site and pasted the code from goodreads into it, and it shows up in my sidebar. You can see how it looks on my site if you scroll down a page: http://wp.intelligentlizard.com/.