Check out this Mathbook | Books - Oscar Levin, Ph.D.'s website

Discrete Mathematics: An Open Introduction writen by Oscar Levin, associate professor at the University of Northern Colorado. He has taught mathematics at the college level for over 10 years and received multiple teaching awards. He received his Ph.D. in mathematical logic from the University of Connecticut in 2009.

Discrete Mathematics: An Open Introduction

Oscar Levin writes,"To support my students in Discrete Mathematics, I wrote a free and open source textbook, Discrete Mathematics: an Open Introduction." The book is now in its 3rd edition and has been adopted at over 30 universities around the world.

You can download the book (completely free), view the online interactive version, or find a link to an inexpensive print copy on the book’s website.

Since Spring 2013, the book has been used as the primary textbook or a supplemental resource at more than 75 colleges and universities around the world (see the partial adoptions list) via Oscar Levin, Ph.D.'s website.

Publisher: Independently published (December 31, 2018) 
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Source: Oscar Levin, Ph.D.'s website