Here are a few Tibetan Texts that might be helpful:

• Achived Texts from the Asian Classics Input Project (including Wylie-Transliteration)
• List of Sera Textbook Definitions (Tibetan)
• Abhidharmakosha – Root Text and Root Text with Dictionary Annotations
• Bodhicharyavatara – Root Text and Root Text with Dictionary Annotations
• Madhyamakavatara – Root Text and Root Text with Dictionary Annotations
• Middling Lamrim (Tsongkhapa) – Root Text

Online research tools and other sources for Tibetan texts and teachings:

• Buddhist Digigal Resource Center (formerly Tibetan Buddhist Resource Center) – Extensive archive of scanned Tibetan texts together with a large database of Tibetan authors and their works
• UMA Institute for Tibetan Studies – Website by Jeffrey Hopkins and others containing various Tibetan textbooks with English translation, and extensive audio section, Tibetan language learning material, and more
• The Buddhist Canons Research Database – Resources for scholars of Tibetan language (Extensive bibliographic database and full-text search for the entire Lhasa Kangyur and Derge Tengyur.)
• Tibetan and Himalayan Library – Many resources about Tibetan Buddhism and Tibetan language by the University of Virginia, incluing sizable parts of the Derge edition of the Tibetan canon.
• The Padmakara Translation Comitee offers a selection of various electronic texts in Tibetan language – some of them for free and some of them for sale. The focus is mostly on Tibetan translations of important Indian treatises as well as material from the Kagyu and Nyingma traditions.