Packtpub book offers and freebies.

Scientific Computing with Python 3
has 17 hours on the clock from the first link.

Covers Scipy, Numpy, etc.

Going for the next 18 hours from the first link up top.

And again, about 18 hours to go.

Python Data Structures and Algorithms by Benjamin Baka
Free for the next ~18.5 hours from the first link up top.

Available again for the next 16.5 hours.

Mastering C++ Multithreading by Maya Posch
is free to download within the next ~20.5 hours and keep forever. Touches on GPGPU amongst other areas. First link in the OP.

Not Packtpub but worth sharing are various books and papers available for free via the following links:

realtimerendering.com/raytracinggems/unofficial_RayTracingGems_v1.3.pdf

realtimerendering.com/raytracing.html#books

link.springer.com/

apress.com/

Enjoy.