The publisher Springer has made a whole raft of their books available as free downloads and this should last until the end of July. This includes a Guide to Scientific Programming in C++ and Systems Programming in Unix/Linux.
There’s also books on networking, cryptography and more.
Alternatively, if you fancy learning x86-64 Assembly Language Programming with Ubuntu then knock yourself out. I used to write a lot of assembler (6502 and z80) a very long time ago. It’s a lot easier these days with debuggers.