This Książka adresowa icon is made in Material Design style developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 4,300 icons, all in the same style and quality.
This is a simple icon meant to represent an address book. It's a rectangle stood on end with rounded corners and the @ sign in the middle. The right hand side of the book is hatched with short lines to represent the rings that hold the pages inside the binder.