Openbiblio is free open source software and can run on very low cost Web hosts. This could be the lowest cost first option for your library.
Drupal 8.6.0 is here. What is different compared to 8.5.6? Drupal 8.6.0 is the upgrade you need when you have images, video, any type of media on your Web site.
Programming in C is difficult and the reward is fast execution of the code. For someone learning C, there is also the reward of excellent brain cell exercise.
Solar power is difficult and expensive but is the better choice for many uses. In sunny Australia, solar power is close to the default power source with power storage the real sticking point. Hydroelectric storage, the current best storage, is coming to towns near you.
SQL databases are still popular for small to medium projects while NoSQL is taking over for large projects. What should you look at when investigating replacements for SQL?
Timeshift is promoted as a way to recover files for a broken system or user directory. I use Timeshift to backup everything so I can restore anything.
Qt and GTK are Graphical User Interface libraries used by software developers to add standard user interfaces to applications on Linux, Windows, and OSX. The reasons for choosing one are loosely based on fashion, practicality, and the details of the various releases of each library. There are also alternatives popping up faster than you can evaluate. There are good arguments for both.