How can I use other font weights (such as Book 300 for Open Sans) in my documents?
Adding Open Sans only allows me to use Normal 400, for example.
It appears that Google docs has very limited functionality for google web fonts right now. For each font you use, you only have the option of the bold weight or normal weight that was set for the web font you selected.
Now that Google Apps Script is implemented in Google Documents, it is possible to create font weights:
I've created an example file for you to play around with: Font Size
See the following answer for instructions on how to implement the script (works the same as in a Google Spreadsheet): Font Size in Google Spreadsheets