Currently it's on sales at Leanpub with $19.80 but apparently the ebook with the Golang code is $39:

https://leanpub.com/build_your_own_database_from_scratch/c/L...

To be honest it seems a bit strange to pay for the code since all the source codes to build the database is literally inside the book.

Can you elaborate on why it was so bad?

Check the reviews online, but generally random bits of Go code are presented with no build-up or explanation. B Trees are implemented immediately and without explanation. The book is incredibly short and seems just like a loose collection of notes, rather than an actual book. Very interested to check out the new version