AMARDEILH, Jean-Pierre, Atlas des orchidées d'Indre-et-Loire, Société Française d'Orchidophile, 2007 – distribution maps (5km squares), brief but excellent descriptions of each species present in the département of Indre-et-Loire, excellent photos. The one criticism I have is that the maps unfortunately don't show the river Claise (although as the Claise valley is the richest orchid area in the Touraine, it is reasonably easy to work out where it is from the concentration of red dots in the southern tip).
BOURNERIAS, Marcel et PRAT, Daniel, et al, Les Orchidées de France, Belgique et Luxembourg, 2ème ed, Société Française d'Orchidophilie, Parthénope Collection, Biotope, Mèze, 2005 - the bible. Photographs, full descriptions, keys, distribution maps to departmental level. Large and heavy - for pouring over at home.
MABEY, Richard, Flora Britannica, Sinclair-Stevenson, London, 1996. Not useful for identification, but fun for quirky pieces of botanical folklore. Large and heavy - for occasional dipping into.
MOULINE, Christian, Les Orchidées du Bassin de la Claise Tourangelle, Les Cahiers de la Claise, Patrimoine Vivant en Claise Tourangelle, France, 1995 - does not include the Brenne (which is in Berry, not the Touraine). Photographs and line drawings, descriptions, distribution maps to 2.5km grid. Slimline (the size and shape of a Michelin Green Guide).
PINET, François, Flore remarquable du Parc naturel régional de la Brenne, Parc naturel régional de la Brenne, Rosnay, 2005 - covers 328 notable plants in the Brenne. Photographs, descriptions, distribution maps. Plants are listed by French name, which is annoying, but information about ecology and location of each species excellent. Good field guide size.
POLUNIN, Oleg, Flowers of Europe: A Field Guide, Oxford University Press, London, 1969. Layout old-fashioned, with photographs separated from text, but coverage excellent and still occasionally useful e.g. line drawings (beware species name changes). Large and heavy i.e. not for taking into the field unless you habitually travel with a pack-horse.
ROSE, Francis (revised and updated by Clare O'Reilly), The Wild Flower Key, Frederick Warne, London, 2006 - only covers those species which also occur in Britain, but contains clear, concise descriptions. Suitable size for taking into the field, but inevitably limited use in France.
Société Française d'Orchidophilie de Poitou-Charentes et Vendée http://www.orchidee-poitou-charentes.org/ - species lists, descriptions with photographs, distribution maps, particularly good section on pollinators, but does not include departéments 36 and 37, which are in Centre.