Many great textbooks and other resources are available for free.
Open Educational Resources (OER) as defined by UNESCO are "teaching, learning, and research materials in any medium - digital or otherwise - that reside in the public domain or have been released under an open license that permits no-cost access, use adaptation by others with no or limited restriction"
Creative Commons licences make copyright material freely available for legal reuse, sharing and remixing.
CC BY (Attribution) - This lets others distribute, remix, adapt and build upon a work, as long as the owner is credited.
CC BY-SA (Attribution-ShareAlike) - This lets others remix, adapt and build upon the owner's work even for commercial purposes, so long as it credited and licensed under the identical terms.
CC BY-ND (Attribution-NoDerivs) - This lets others use the work for any purpose including commercially, with credit, however it cannot be shared in adapted form.
CC BY-NC (Attribution-NonCommercial) - This lets others remix adapt, and build upon the work non-commercially with credit, but the works do have be licensed on the same terms.
CC BY-NC-SA (Attribution-NonCommercial-ShareAlike - This lets others remix, adapt and build upon work commercially, as long as it is credited and licensed under the same terms.
CC BY-NC-ND (Attribution-NonCommercial-NoDerivs) - Only allowed to be downloaded and shared with credit but can't be changed or used commercially
You can search for Open Education Resources by selecting "Open Access" in the Refine my Results options in Library Search
Principles of Finance by 
A New History of Management by 
Sustainable marketing and strategy by