I would definitely go for the Brendan Gregg book. He's a long term BPF contributor and his book seems much more in-depth compared to the Oreilly one which only has 200 pages compared to >700, and if you look at the Oreilly book's TOC it appears it's trying to cover everything BPF (not just tracing) in just 200 pages. Definitely Brendan Gregg book's seems a better choice as far as I can tell.