A page called Thesaurus musicarum latinarum, which contains the latin texts of the majority of treatises by music theorists from th 3rd century to the 17th century, including for example, Boecius, Isidorus from Sevilla, or Gaffurius.
Extract of the text "Arte de tañer fantasia" (1565) by Tomas de Sancta Maria.
Here is the book by Agazzari Del Sonare Sopra’l Basso Con Tutti Li Stromenti E Dell’ Uso Loro Nel Conserto (1607), one of the first texts of the musical baroque period devoted to the basso continuo (here in the original italian, and translations into English and German).
And here the preface by Viadana to his book Li Cento Concerti Ecclesiastici (1602), another important text about the basso continuo.
And this is a link to the Tratado de glosas, by Ortiz, that is, to the "Prefacios del Tratado de Glosas sobre cláusulas y otros géneros de puntos en la música de violones", Roma 1553, by Diego Ortiz (c. 1519-c. 1570).
A page devoted to Folia, to the Folia genre, to all Folias along the history.
And here there is a link to the Web page of ESMUC devoted to early music. Even though most information is only accesible with a password, the part of free access is quite interesting.