The following are excellent aides in studying Onkelos, the Aramaic translation of the Torah:
- Israel Drazin and Stanley M. Wagner's book Onkelos on the Torah: Understanding the Bible Text. I only have the Bereshit volume of this English translation of Onkelos (there is also a Shmot edition), but it is very helpful for my work. It includes an English translation of the Torah text, based on Onkelos. Where Onkelos deviates from the plain meaning of the text, the English words are highlighted in bold. There is also an extensive commentary, explaining the reasons behind the Onkelos translation, as well as a comparison with other translations and commentaries.
- Dov Rappel's work "Targum Onkelos". This Hebrew book is a complete study of the approach of Onkelos, with such chapters as "The Theology of Targum Onkelos", "Targum Onkelos and the Midrash", "The Halacha and Targum Onkelos" and "The Literary Aspect of Targum Onkelos". While the book is meant to be studied straight through, there is an index of all verses mentioned, so you can find out if there is any interesting commentary on a particular translation. Dov Rappel was a professor at Bar-Ilan university, and a member of Kibbutz Yavne. I sat behind him in the Beit Knesset when I lived there, and found him to be a tremendous talmid chacham and a uniquely humble individual. He died in 2003 at the age of 87.
- Yein HaTov by R' Alter Vein. This book (as described here) "highlights every place where a Targum differs from the literal translation, translates the Targum back into Hebrew and sometimes explains the significance of the difference." A good resource, but it would be nice if the number of the chapter, as well as the name of the parasha appeared on every page.