If you don't want to manage individual inventory items in Book Keeper, you can enter stock value manually using Stock Account. Follow these steps:
1. Go to Company Settings > enable 'Accounts Only'
2. Go to Inventory > Stock Account
3. Create a stock account, enter its name, value on the selected date.
4. You can enter multiple values of a stock account for different dates. All will reflect in financial reports.
Here's video tutorial for your reference: