This is a User Manual Filled icon. We've made it in iOS style, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 12,800 icons, all in the same style and quality.
This icon for User Manual looks like an open book. The cover of the book looks like a rectangle with rounded edges, and two pages are visible, separated by a line in the middle of the pages. On the page on the right, there is a large lower case letter i.