Obviously we cannot explore a country without maps and in France (and most of Europe) that means Michelin. The Green Guide recommends the following maps:M82 and 83 (at a scale of 1/200
000 and a more regional maps M235 and M240 also at a scale of 1/200 000.
I could not find these maps on the Michelin Map website and it is possible that they have been replaced by new editions. The closest map on the web site is M11344 (Aude, Pyenees-Orientale) at a scale of 1/150 000 (1cm=1.5km). The map covers all of the Canal du Midi, though I wish it went farther north to cover the National Parks. Maps can be generated, and viewed on the Michelin Web site and below are two examples. We pick up boats at Castlenaudary (in the centre of MAP 1 and travel West to Toulouse or East to Carcassonne. Map 2 shows the canal section further east, between Carcassonne and Beziers.
Map 1: Toulouse to Carcassonne
MAP 2: Carcassonne to Beziers.

I am sure there will be other local maps available in France, once we get there.
An excellent source of on-line maps is
MAPQUEST. Detailed maps of France are available and I was even able to locate a small village where the only Minervois wine available is Nova Scotia is bottled.
Michelin. Languedoc, the area of France through
which Canal du Midi winds its way, is not very familiar so I have looked
for some background and reference material. The first find, and a real
"must" is Michelin's
Green Guide to Languedoc, Roussillon Tarn Gorge.
[On Michelin web site scroll down to "Regions of France" and
select Languedoc from the pull down menu.] The guide is full of factual and
fascinating information about places along the Canal as well as the rest of
the region. Essential reading!
The Rough Guide to Languedoc & Roussillon by Brian Catlos. 416 pp - 27 maps. Recommended by several travellers on their personal web sites. "Rough Guides" foster a community of young travellers and encourage posting personal travel diaries. There are five accounts of visits to Caracassonne and they make interesting reading.