Public Domain = Anyone can use it for any purpose; it's free to use without restriction.
Royalty Free = Someone else owns it, so you can't own it; but, you can borrow it.
(Public domain: free within the U.S.)
Digital Public Library of America
https://dp.la/
Feedbooks
http://www.feedbooks.com/publicdomain
The Library of Congress
https://archive.org/details/library_of_congress?&sort=-downloads&page=4
Librivox
http://www.librivox.org
Project Gutenberg
http://www.gutenberg.org
Rare Book Room (Public domain: free within the U.S.)
http://rarebookroom.org/
(Royalty-Free)
Kobo
https://store.kobobooks.com/en-us/list/kids-teen/KXcdXaYEQUyXx_o_Ur-aww
Manybooks
http://manybooks.net/genres/teen-young-adult
University of Pennsylvania
http://onlinebooks.library.upenn.edu
(Mixed:
Each item is individually labeled, so make sure you read the small print!)
Open Library
https://openlibrary.org/
World Digital Library
https://www.wdl.org/en/
Creative Commons Licenses =
Provides an excellent summary 
of access and use definitions and policies.
last updated 6/7/2018 by Arlen Kimmelman, CRHS Library