Goal for this part
Collect book details with Entry widgets, validate title/author/copies, and show success or error with messagebox. Nothing is saved to disk yet—validation comes first.
Concepts: StringVar and validation
StringVar holds the current text of an entry. On submit, strip whitespace, require title and author, and ensure copies is an integer between 1 and 50. Desktop forms still need the same discipline as web forms: never trust raw input.
How to run it and what you should see
Submit an empty title and confirm an error dialog. Fill valid fields and confirm the “Looks good” message.