Some of the best references I have found has been via Google Books. From Wikipedia …
Google Books is a service from Google Inc. that searches the full text of books and magazines that Google has scanned, converted to text using optical character recognition (OCR), and stored in its digital database. Books are provided either by publishers and authors, through the Google Books Partner Program, or by Google’s library partners, through the Library Project. Additionally, Google has partnered with a number of magazine publishers to digitize their archives.
It might seem to be a bit OCD to research subjects like retaining walls when modeling – it’s just a model, right? Just build and let go might be the first response. For me learning the correct way to build a retaining wall lets me model something I am happy with when completed.