Features:

• CK-12 is probably the biggest open content provider for K-12 STEM content.
• CK-12 provides standards-aligned teaching resources. Content is mapped to a variety of levels and standards, including common core.
• The most incredible feature of CK-12 is its customizable textbook.You can customize a textbook (called “Flexbook”) for your students to use. You can start from scratch or build from anything in the FlexBooks library.
  
• Watch this video to learn more about the Flexbook feature of CK-12

License Setting

• All CK-12 courses are licensed under CC BY NC 3.0.
• This license lets you remix, tweak, and build upon the original work non-commercially, and although your new works must also acknowledge you and be non-commercial, you don’t have to license your derivative works on the same terms.

Sample Attribution

CK-12 made it clear about how they wish to be cited. It is a little bit unconventional and I think it is best for you to visit their Attribution Guidelines and learn their requirements.