Have you seen the
www.viafrancigena.me forum? - a sister site to this one. A good place to post any specific questions about the Via Francigena and read a little of other people's experiences. There is also a very active Facebook group dedicated to the Via Francigena where a number of people have posted frequent updates, comments and links to personal blogs. A useful resource:
https://www.facebook.com/groups/19899007360/
The critical point for timing your walk is the Grand Saint Bernard Pass between Switzerland and Italy. It is closed to walkers by snow for much of the year and is reliably open only from June to September. Most people take somewhere between 80 and 100 days for the whole walk, and the Pass is close to the halfway point. I think that unless you expect to travel very slowly there would be little point in leaving Canterbury before mid-April or after mid-August. Apart from the Grand St Bernard Pass the route is passable for all the year.
Many French campsites have very short summer seasons and may not be open before June or even July, and may close again in September. That may be a difficulty if you plan to walk in spring unless you are prepared to wild camp ("bivouac", "camping sauvage"). I did this for most of my nights in France but I walked a different route to Besancon before joining the waymarked route there. I have read several blogs by people who have done so without major problems. Once in Italy there is a fairly good and growing network of pilgrim accommodation at low cost which makes camping unnecessary. There are also very few official campsites along the Italian stages. I carried a very small one-person tent which weighed about 1kg. Essentially a large hooped bivi-bag. Up to you how you balance space and comfort against the weight and bulk of the packed tent.
I did not use any guidebooks so cannot recommend any. In France and Switzerland I navigated using canal maps, pages from a road atlas, and mobile phone mapping from the French government map agency: m.geoportail.fr If you read the posts on the VF Facebook group you will find a number of opinions - very varied of course - on the merits and faults of the English guidebooks
Once in Italy there are excellent maps and accommodation lists free online at
http://www.visit.viefrancigene.org/en/