"Open-access (OA) literature is digital, online, free of charge, and free of most copyright and licensing restrictions. What makes it possible is the internet and the consent of the author or copyright-holder." -- Peter Suber, A Very Brief Introduction to Open Access

Characteristics:

• Freely accessible to all, without charge, institutional authentication requirements, or licensing fees
• Copyright and/or licensing restrictions on use are mostly or wholly absent
• Compatible with peer review
• Funding models that do not depend on reader subscriptions or pay-per-use or per-article fees
• Exists as either archival repositories, or fully-fledged peer-reviewed journals
• Focuses on royalty-free literature--that is, work where the author does not receive monetary compensation (though the other rewards of scholarship do accrue!)

Some context:

• Widespread Internet adoption makes publication and access quicker and easier than ever
• Journal subscription prices have far outstripped inflation, while library budgets have remained flat or even declined
• Journal titles have proliferated at an exponential rate

These collections of free and/or open access e-books come from a wide variety of sources, mostly academic.