Ugh, there's a basic PostgreSQL book listed on there, which (from the Amazon reviews) seems to be the worst possible pick. eg it's rated as 1 star. :(

https://www.amazon.com/PostgreSQL-Development-Essentials-Man...

That's really unfortunate, as there are a tonne of PostgreSQL books around, many of which are very good. eg:

https://www.amazon.com/Practical-SQL-Beginners-Guide-Storyte...

https://www.amazon.com/PostgreSQL-Running-Practical-Advanced...

Conversely, the MySQL book right above it in the list seems to be one of the highest rated ones on Amazon:

https://www.amazon.com/Learning-MySQL-MariaDB-Heading-Direct...

As GitHub uses MySQL internally, this seems like a fairly plain attempt at GitHub being dodgy.