Salam,
I have noticed the habit of licking a finger to turn a page of a book, or sort through any paper items (such as money) prevalent amongst some folk from the Indian subcontinent. It seems this practice was also noted in the Arab world centuries ago and warned against by Muslim scholars.
The following is from Shaykh Muhammad bin Abd al-Rahim al-Mulla (d.1100 hijri) in his work Miftah al-Qurb p.219:
Even though saliva is pure it is however regarded as disgusting. It was due to this that some scholars severely rebuked those who soil the pages of a mushaf (copy of the Quran) with saliva. This is also the case if it was done to make turning the pages of the Qur’an easier…
Ibn al-Hajj said in al-Madkhal: It is unlawful to wipe a slate containing Quranic text or even a part of it with saliva, and it is incumbent for the teacher to forbid the students from doing this.
The prohibition of wiping out the name of Allah with saliva has been cited in the work Bahr al-Raiq.