Hi,
I meant to respond to this sooner so my apologies if you have already started your research. I hope my tardy reply is still helpful.
I am currently researching child passenger safety here in the UK during the late 1970s and early 1980s; the same period in which the UK's first seatbelt law was passed. Like you, I found actual books on the subject rather scarce so I can completely understand why locating books on a specialist area such as extended rear facing would seem nearly impossible!
In my early research I did find the following books which I hope will be of help to you. The two books I recommend you look at are:
Sanders, Robert S Jr., Dr Seat Belt: The Life of Robert S Sanders, M.D. Pioneer in Child Passenger Safety, (Murfreesboro: Armstrong Valley Publishing Company: 2008) – ISBN: 978-1-928798-09-5
Packer, Jeremy, Mobility Without Mayhem: Safety, Cars & Citizenship, (Durham: Duke University Press: 2008) – ISBN: 978-0-8223-3963-2
The first book, Dr Seat Belt, is a treasure trove of interesting and useful information however it does have some drawbacks. Foremost, the editing is relatively poor. The chapters are somewhat erratic in structure and consequently the book does not read particularly fluently. In spite of this rather obvious shortfall, it is accessible and relatively simple to read thanks to the unfussy and lucid writing as well as the passionate authorship of Dr Sanders close friends and relatives.
The second book, Mobility Without Mayhem, is a much more thorough and polished work, with a much larger breadth of information. However, due the density of the ideas and concepts (which are most interesting) it is beyond the simple evening-catch-up type of book. It is absolutely worth the money and is very well written. I have not checked, but it is likely more readily available than the Sanders book.
I should point out that neither of these books makes specific reference to extended rear facing but rather to a much broader spectrum of child passenger and road safety. They could well be useful and I very much hope they are, but (I’m sure you’ve heard this a thousand times over!!) I wouldn’t cite them unless they help bolster your argument and benefit the paper.
I hope that is of some help to you and good luck with the paper!
Gary.