Since a few days (September 2019 as written on the side was/is wrong), the 3rd book of the GameAI Pro series is available for free on their website. As with the past 2 books, it’s a nice and easy to browse list of the actual book content converted into single PDFs for each article, so quite handy to just pick what one’s interested at the moment.
The previous 2 books are still available too, so there’s a lot of valuable stuff to read.