There are tons of free books, but here are some of the most well known/essential books for any collection. (Will add more over time.):
Classics:
Frakenstein: Frankenstein - Kindle edition by Mary Wollstonecraft Shelley. Literature Fiction Kindle eBooks Amazon.com.
Twenty Thousand Leagues Under the Sea: Amazon.com Twenty Thousand Leagues under the Sea eBook Jules Verne Kindle Store
The Time Machine: Amazon.com The Time Machine eBook H. G. (Herbert George) Wells Kindle Store
The War of the Worlds: Amazon.com The War of the Worlds eBook H. G. (Herbert George) Wells Kindle Store
Pride and Prejudice: Pride and Prejudice - Kindle edition by Jane Austen. Literature Fiction Kindle eBooks Amazon.com.
Historic:
The US Constitution:
Amazon.com The United States Constitution eBook USA Kindle Store
The Communist Manifesto: The Communist Manifesto - Kindle edition by Friedrich Engels, Karl Marx. Politics Social Sciences Kindle eBooks Amazon.com.
The Wealth of Nations: An Inquiry into the Nature and Causes of the Wealth of Nations - Kindle edition by Adam Smith. Literature Fiction Kindle eBooks Amazon.com.
Notebooks of Leonardo Da Vinci: The Notebooks of Leonardo Da Vinci Complete - Kindle edition by Leonardo da Vinci, Jean Paul Richter. Literature Fiction Kindle eBooks Amazon.com.
The Art of War: Amazon.com The Art of War eBook Sunzi, Lionel Giles Kindle Store
Religion:
The Holy Bible: The Holy Bible, English Standard Version (with Cross-References) - Kindle edition by Crossway Bibles. Religion Spirituality Kindle eBooks Amazon.com.
The Quran: Quran A Simple English Translation (Goodword ! Koran) - Kindle edition by Maulana Wahiduddin Khan, Goodword, Goodword Maulana Wahiduddin Khan. Religion Spirituality Kindle eBooks Amazon.com.
The Book of Mormon:
Book of Mormon - Kindle edition by The Church of Jesus Christ of Latter-day Saints. Religion Spirituality Kindle eBooks Amazon.com.
Almost Free
The Complete Works of Shakespeare (99 cents): The Complete Works of William Shakespeare (37 plays, 160 sonnets and 5 Poetry Books With Active Table of Contents) - Kindle edition by William Shakespeare, William James Craig, Henry M. Piironen. Literature Fiction Kindle eBooks Amazon.com.
Classics:
Frakenstein: Frankenstein - Kindle edition by Mary Wollstonecraft Shelley. Literature Fiction Kindle eBooks Amazon.com.
Twenty Thousand Leagues Under the Sea: Amazon.com Twenty Thousand Leagues under the Sea eBook Jules Verne Kindle Store
The Time Machine: Amazon.com The Time Machine eBook H. G. (Herbert George) Wells Kindle Store
The War of the Worlds: Amazon.com The War of the Worlds eBook H. G. (Herbert George) Wells Kindle Store
Pride and Prejudice: Pride and Prejudice - Kindle edition by Jane Austen. Literature Fiction Kindle eBooks Amazon.com.
Historic:
The US Constitution:
Amazon.com The United States Constitution eBook USA Kindle Store
The Communist Manifesto: The Communist Manifesto - Kindle edition by Friedrich Engels, Karl Marx. Politics Social Sciences Kindle eBooks Amazon.com.
The Wealth of Nations: An Inquiry into the Nature and Causes of the Wealth of Nations - Kindle edition by Adam Smith. Literature Fiction Kindle eBooks Amazon.com.
Notebooks of Leonardo Da Vinci: The Notebooks of Leonardo Da Vinci Complete - Kindle edition by Leonardo da Vinci, Jean Paul Richter. Literature Fiction Kindle eBooks Amazon.com.
The Art of War: Amazon.com The Art of War eBook Sunzi, Lionel Giles Kindle Store
Religion:
The Holy Bible: The Holy Bible, English Standard Version (with Cross-References) - Kindle edition by Crossway Bibles. Religion Spirituality Kindle eBooks Amazon.com.
The Quran: Quran A Simple English Translation (Goodword ! Koran) - Kindle edition by Maulana Wahiduddin Khan, Goodword, Goodword Maulana Wahiduddin Khan. Religion Spirituality Kindle eBooks Amazon.com.
The Book of Mormon:
Book of Mormon - Kindle edition by The Church of Jesus Christ of Latter-day Saints. Religion Spirituality Kindle eBooks Amazon.com.
Almost Free
The Complete Works of Shakespeare (99 cents): The Complete Works of William Shakespeare (37 plays, 160 sonnets and 5 Poetry Books With Active Table of Contents) - Kindle edition by William Shakespeare, William James Craig, Henry M. Piironen. Literature Fiction Kindle eBooks Amazon.com.