Here is a selection of botany field guides that I can't get by without, and so will be in the book box for France.
The Wildflower Key, Francis Rose, updated by Clare O'Reilly, 2006, Frederick Warne. One of the reviews for this revised edition when it came out said something like 'you have more chance of arriving at a correct identification for a wild flower with this book than any other'. For a key based book it is extremely user-friendly and the illustrations are very good. Clare and her husband John have a Botany Blog, and Clare was one of my university tutors, so I know how much care and effort she would have put into the revision of this excellent book.
Les Orchidées de France, Belgique et Luxembourg, 2nd edition, Société française d'Orchidophilie, R Duget & F Melki, Parthenope Collection by Biotope, 2005. The bible (and it's big enough). Extremely comprehensive and thorough; etymology of scientific names; all currently known naturally occurring hybrids listed (the field botanist's bane); distribution maps (to départment level); good photographs, which include a note on where taken. There is a charming little anecdote in the preface in which the editor relates meeting an English couple in the field, première édition in hand. The introductory section includes a photograph of two members of the société laying on the ground, terrorising some poor little orchid with their macro lenses, with a wry caption to the effect of 'it's important not to get so excited you squash the plants', and a unexpected photo of orchids merrily flowering in the snow, once again surrounded by half a dozen oohing and aahing members of the société.
Flore remarquarble du Parc naturel régional de la Brenne, François Pinet. Covers the rare and endemic species in the park only. An extremely good introductory section covering geology, climate and hydrology, which I have previously cribbed for the blog here, here and here. The information about each plant is good, right down to the number of places each species occurs in the park, and where. It has a major and extremely annoying fault though - the plants are listed in alphabetical order of French common name. This means that plants in the same family or even genus, which may be very similar to one another, are not always next to each other in the book, and if you are not familiar with their vernacular name, you may not even realise a plant's lookalike is even in the book.
Photographic Field Guide: Trees of Britain and Europe, Bob Press and D Hosking, New Holland, 1993. One of those surprisingly good cheap field guides - pretty good photos, brief but professional descriptions, line drawings to clarify when there are lookalikes.
Guide Nature de la Brenne, Jacques Trotignan, Tony Williams et al, 2nd edition, LPO & WWF-France, 2001. An excellent overview of the park and what can be seen there. The only criticism I have is that in order not to frighten people with long unpronounceable scientific names they have chosen to go with French vernacular names only, even in the checklists at the back. It makes it unnecessarily hard work cross referencing to other field guides.
Les Orchidées du Bassin de la Claise Tourangelle, Christian Mouline, Les Cahiers de la Claise, 1995. A little gem of a book, with distribution maps pinpointing orchid plants in the Claise Valley down to a few metres.
Umbellifers of the British Isles, BSBI Handbook No 2, TG Tutin, 1980. It may look old fashioned and have only line drawings, but it is the only way to accurately separate your Queen Anne's Lace from your Wild Carrot or your Cow Parsley from your Hemlock in the field. The only disadvantage in France is that you are dealing with more species than are present in the British Isles. The book teasingly tells you how many more species, but does not name or illustrate them.
Colour Identification Guide to the Grasses, Sedges, Rushes and Ferns of the British Isles and North West Europe, Francis Rose, Viking, 1989. Easy to use guide for a difficult group of plants. Luckily, the boundary of the area it covers is the Loire Valley.
Sedges of the British Isles, BSBI Handbook No 1, AC Jermy et al. The long awaited 3rd edition, 2007.
Flowers of Europe: a field guide, Oleg Polunin, OUP, 1969. Another oldie but goodie, it is still the current text.
(These last two I don't actually have yet, but I am ordering the Sedges and my parents are bringing the Polunin - a legacy from my father's aunt, who died last year.)