As turtleSpaces is based on and compatible with Apple Logo II / LCSI Logo II from the 1980s, there are plenty of freely-downloadable books available online that will help you learn the basics of the Logo language while also exploring computing history!
Here are a few to start:
Apple Logo II Reference Manual: https://archive.org/details/Apple_Logo_II_Reference_Manual_HiRes
Discovering Apple Logo: https://archive.org/details/Discovering_Apple_Logo_David_Thornburg/
Introducing Logo: https://archive.org/details/tibook_introducing-logo/
Primarily Logo: https://archive.org/details/primarilylogo/
There are also several dozen editions of the Logo Exchange, a newsletter published in the 1980s and 1990s available here: https://el.media.mit.edu/logo-foundation/resources/nlx/index.html
The Great Logo Adventure is mostly relevant: http://ftp.cs.duke.edu/courses/cps108/spring00/projects/slogo/book/
You may be asking yourself, “How do these old books help me create modern 3D video games?” and the answer is simply that these books teach you the equivalent of primary-school English – the basic grammar and syntax of the Logo language. turtleSpaces builds on the basic Logo language by adding a number of additional words (called ‘primitives’) to the vocabulary that provide additional functionality to the coder. It also adds some additional grammar styles while not disrupting the original grammar rules – why re-invent the wheel when you want to build a sports car?
Going through a few of these books and then looking at the examples provided in the /examples folder will be enough for many if not most budding turtleSpaces coders to start writing 3D programs.