For many (though not all) of my e-books, I have the option to put them on sale for seven days, several times a year. One of those weeks is starting today, June 13, 2015, I think at 8 a.m. (If you are reading this after June 20, 2015, this particular sale is over but the regular prices of all of these books are still affordable.)
These are Kindle countdown sales. The way they work is that the prices of the books are at their very lowest (99 cents) on the first day, then gradually increase over the course of the week to their usual list price. The sale is ongoing at both the US and the UK Amazon sites.