I've made a small app called ZotShelf for Android devices. It extraxts and shows the covers of the epubs or pdfs of a collection or tag in a Zotero account. The book covers are clickable leading to the book in the web library (and in time the Zotero app when it supports internal zotero-links). The app is meant as an add-on to Zotero, making Zotero more usable for reading books and to make the books stand a bit more out than they do today in the list view. (The motivation for making the app was moving from Kindle which had such widgets.)

You can find the app here:
https://play.google.com/store/apps/details?id=oyvindbs.zotshelf

If you have any issues with the app or any feature requests please comment here or send me an e-mail at appbugge@gmail.com (The widget feature may be more buggy as I haven't tested it much as I use Boox devices)