The most expensive free book I got so far is this monster, which I got about a month ago shortly after it was published. Retails for $500.00.
Safety and Reliability – Safe Societies in a Changing World
Proceedings of ESREL 2018, June 17-21, 2018, Trondheim, Norway
Print Length: 3234 pages
Publisher: CRC Press; 1 edition (June 15, 2018)
It's also available for free (as a pdf) directly from the publisher, which is a reputable publisher of technical books.
https://www.crcpress.com/Safe-Societies ... 0815386827
I now have both, the kindle and the pdf. In fact, I suspect the kindle edition is merely the pdf in a kindle wrapper, since the kindle edition text does not reflow like a normal kindle book does.
Previous years proceedings are $494.99 for the pdf. I was not able to find any of those previous years for free, not even on amazon. Thus I am predicting that the 2018 edition will also be $500 by this time next year, when I hope to get the 2019 edition for free.
That's a lot of pages. Thumbing through the table of contents, there is a lot of interesting stuff in there — and I do mean a lot, since the average chapter is maybe ten pages — and I can now say I have actually read one of my free kindle books. Well, one chapter anyway, called, "Safety analysis of autonomous driving using semi-Markov processes". I was curious to know how they used Markov models and what their conclusion is. I was surprised to learn they recommend skipping level three autonomy mode and going straight to level four. Makes sense to me.