What are the texts that contain words which can be attributed directly to the Buddha? If I understand correctly, the Buddha never wrote anything but some Buddhist texts quote him by describing meetings or situations where he was present. Is there a comprehensive list of all scripts where he is directly and textually quoted?
The Pāli Canon (Pali: Tipitaka) is the standard collection of scriptures in the Theravada Buddhist tradition, as preserved in the Pāli language. It is the most complete extant early Buddhist canon. It was composed in North India, and preserved orally until it was committed to writing during the Fourth Buddhist Council in Sri Lanka in 29 BCE, approximately four hundred and fifty four years after the death of Gautama Buddha. 


See Pāli Canon on Wikipedia, also called the Tripiṭaka or Tipitaka, meaning "Three Baskets." I would recommend these books as a good reference to access original Buddhism. 
Here is an example from the Mulapariyaya Sutta, Majjima Nikaya section of the Pali Canon.


  I have heard that on one occasion the Blessed One was staying at Ukkattha, in the shade of a royal Sal tree in the Very Blessed Grove. There he addressed the monks, "Monks!"
  
  "Yes, lord," the monks responded.
  
  The Blessed One said, "Monks, I will teach you the sequence of the root of all phenomena [or: the root sequence of all phenomena]. Listen &amp; pay close attention. I will speak."
  
  "As you say, sir," they responded.


This is an example of how each "Sutta" starts.