For me worked well solution:

1) Create library notebook. For example - "Lib" with any functions/classes there (no runnable code).

2) Create main notebook. For example - "Main"

3) To import into main all classes & functions from Lib to Main use command:

%run "./Lib"

(this will works like: from Lib import *)

4) After that you can call any functions/ use classes that used in Lib from Main notebook.

PS:

1.1) recursion enabled - i.e. you lib notebook may contain code that runs any other notebooks the same way

1.2) for reload changed code from Lib module - just re-run command %run "./Lib"