Musar do Izajasza 1:11

The subject of the three gifts the Jewish people received must be understood on two levels. One level is the manifest one, i.e. נגלה, a gift which everyone can see as having been received. The other level is what we call the נסתר the hidden aspect of these gifts. We find that the word Tov is spelled with different vowels on different occasions. Most of the time it is spelled with the vowel חולם i.e. Tov, whereas sometimes it is spelled with the vowel שורוק i.e. Tuv.