Until recently, there have been no really good, current books on Roman roads in Britain. The standard work by Margary (first edition in two volumes in 1955 and 1957; second edition 1967; third edition 1973) is now out-of-date and in need of revision; nevertheless it is still an invaluable work of reference and is occasionally available in one of its three editions on the secondhand market. The third edition route descriptions have been used as the basis of the website accompanying this book (see below).
The most succinct and readily-available study of the subject of Roman Roads in Britain is the Shire Archaeology volume by Davies (2009), replacing the dated volume by Bagshawe (1979). Both are useful as an introduction. Davies has also produced a more detailed study of Roman Roads in Britain (2002) and a diachronic study (2006) of British roads, ranging from prehistory to the modern day. For a refreshing alternative view on the surveying and layout of Roman roads, see Poulter (2010).
There are considerably more accounts of the roads of individual regions, usually the results of the labours of individuals. Noteworthy here are the following studies of Roman roads in Dorset by Field (1992), the northern Lake District by Allan (1994), and North Wales by Waddelove (1999).
A medieval perspective on the road system of Britain can be found in another Shire Archaeology volume, this time by Hindle (1998), whilst a useful diachronic regional study is that of Dodd and Dodd (1980), examining the roads of the Peak District from prehistory to modern times.
An invaluable comparative study of military road building is that of Taylor (1996) which explores the work of Wade, Caulfeild, and their colleagues.
Two main strands of mapping are of use for the study of the known (or strongly suspected) Roman roads of Britain. Access to a good library will be necessary to consult these.
Arguably the most important are the First through Fourth editions of the Ordnance Survey’s Roman Britain maps, now out of print (the 6th edition is, unfortunately, markedly inferior to these and best avoided for serious study of Roman roads).
Then there are the two volumes of the Tabula Imperii Romani (TIR 1983; 1987), one each for the north and south of Britain, also now out of print.
Ironically, the work of Margary’s predecessor, Codrington (1918), is now more accessible than his illustrious successor, thanks to the text of his book being available on the World Wide Web as both a scan and a transcription. The transcription, undertaken by Bill Thayer, will be linked to maps of the roads, thus making it an extremely dynamic resource: http://tinyurl.com/Codrington-Roman-Roads
A scan in the form of a PDF and computer OCR transcription is also available: https://archive.org/details/romanroadsinbrit00codruoft
A further useful website is that of the Birmingham Roman Roads Project which details the progress of its members in increasing knowledge of the road system in the West Midlands: http://www.brrp.bham.ac.uk
The NEHHAS website devoted to their Roman road project is accessible at: http://www.hants.org.uk/nehhas/RRAbstracts.html
A particularly useful resource is the complete coverage of mainland Britain by six-inch First Edition Ordnance Survey maps provided by British History Online: http://www.british-history.ac.uk/map.aspx?pubid=270
Less useful at the time of writing is the Old Maps website which provides the same thing but in a more constrained (and somewhat clunky) interface: http://www.old-maps.co.uk
Both of these are of inferior quality (they use the same, rather dated, monochrome scan) by comparison with the greyscale coverage of Scotland provided by the National Library of Scotland’s Map Library Digital Map resource: http://maps.nls.uk
Finally, details of scheduled ancient monuments (including lengths of Roman road) are accessible on an OS map base by means of Defra’s MAGIC website: http://www.magic.gov.uk/