The Kitab al-aghali. (The Book of Errors) by Abu Abd al-Ra.man al-Sulami (d. 412/1021) elucidates a number of errors in both thought and practice that a traveler of the Sufi path might fall into. This study presents a critical edition of the Arabic text of Kitab al-Aghali. produced from four manuscripts (three of which were previously uncatalogued), a detailed synopsis of the book, and addresses questions of authorship and the relationship between Kitab al-aghali. and the Kitab al-luma of Abul Na.r al-Sarraj (d. 378/988). The main manuscript used in forming the critical edition treats the Kitab al-aghali. as a section of another Sulami work previously thought lost - Mi.an mashayikh al-.ufiyah (The Trials of the Sufi Masters) - a critical edition of which is included as an appendix.