A few comments on Sefer ha-Gan are therefore in order. It is a biblical commentary from 13th century France, attributed to Aharon ben Joseph ben Aharon (on whom see Avraham Grossman's article in the Aharon Mirsky festschrift, Be-Orah Madda). I once looked at the manuscript of Sefer ha-Gan and found that it is an Ashkenazic manuscript of the Pentateuch, with Massorah and Rashi in the margins. There are also little circles with micrographic writing inside them - that's Sefer ha-Gan. It's amazing that Orlian has managed to produce a book out of such daunting materials.
The field of Ashkenazic biblical commentary is sorely under-explored. Until recently, the only studies were those by Yitshak Shimshon Lange of Zurich from the 1970s. Now we can add Joy Rochwarger's MA thesis from Touro College on Pa'aneah Raza (Jerusalem 2000) and Hazoniel Tuito's doctorate on Minhat Yehudah, completed at Bar Ilan University in 2004. Hopefully, Prof Ephraim Kanarfogel's next book will provide a wider perspective on this literature, its cultural context and exegetical agenda.
UPDATE: Prof. Kanarfogel pointed out to me that Grossman's article is about a different person by the same name.