Imam Muhammad Zahid al-Kawthari (rahmatullahi alaihi), one of the greatest Hanafi savants of the past century, once wrote a short, comprehensive, and rare treatise on the subject of tawassul, which I have uploaded here for the benefit of all.
If the font size is too small, simply copy and paste the document onto Microsoft Word and then set the view size to 150%, or simply enlarge the text to a readable size.
Ṭaḥāwī 