I'm always looking for sources of good, current books and this one look very good.
They offer free PDF downloads of textbooks OR you can purchase hard/soft physical books at fairly large discounts to bookstore/college prices, for the same books.
https://openstax.org/DO ADD these to your prepper book archives, either as digital (PDF) or paid physical books.
On warning; the website is very particular about the browsers which may be used to view their pages.
My Firefox fork, Palemoon, is not on their list of "acceptable" browsers, but I was able to use the current Firefox Ver 60.0.2 to access their site and download many books in PDF format, for free.