If you switch to the barter section of the trade window, can you add skillbooks on the merchant side and buy them that way?
If you open Party management from the in-game menu and swaps who is controlling which character, do you still have this problem, or does if follow the character so your brother can not buy skillbooks?
If you load an older save, if available, can you buy skillbooks again?
Try clearing the PS4 cache: press and hold the PlayStation button on the controller, select 'Turn Off PS4', wait another 30 seconds after it finishes shutting down, and start it again.
If older saves are ok and this just happens in your recent saves, if possible please email supportdos2@larian.com with your latest save (or a Dropbox or Google drive link, etc), as well as the most recent save where you can buy skillbooks, and oldest that you can not (if you played for a while after this started).
The save(s) would need to be transferred to a USB flash drive first. In the Saved Data in System Storage section of the Settings menu, click Copy to USB Storage Device, select the relevant save(s) and Copy. From there you can zip the files on a computer, then copy to email or upload somewhere and provide a link.