OP here.
A few of you have asked why we made these problems free. The answer is twofold, simple, and maybe even a bit underwhelming:
1) We want people to read the book (To wit, we've also made 9 chapters of the book free: http://bctci.co/free-chapters)
2) We want people to use interviewing.io
In my career, I've written a lot of stuff about hiring, and I've shared a lot of interview-related materials (e.g., full length interview replays). I hate paywalls for content, and you probably do too... and I have never regretted making it free. In my experience, putting good stuff out there is the best way to market to an eng audience.
1) We want people to read the book (To wit, we've also made 9 chapters of the book free: http://bctci.co/free-chapters)
2) We want people to use interviewing.io
In my career, I've written a lot of stuff about hiring, and I've shared a lot of interview-related materials (e.g., full length interview replays). I hate paywalls for content, and you probably do too... and I have never regretted making it free. In my experience, putting good stuff out there is the best way to market to an eng audience.