Adrien, of course, different books cover differ groups of organisms and do so to level levels. For a comprehensive manual, see Flora Novae Angliae (Haines 2011). It treats all tracheophytes growing wild in New England. A tracheophyte is a vascular plant (excluding mosses). For a more amateur guide to wildflowers, see WildFlowers of the Field and Forest (Clemants and Gracie). Of course, this still leaves out the ferns, though the new Peterson guide is fully updated for the ferns and fern allies (including new images and new taxonomy). The woody plants are not treated well in any guide (as far as I'm concerned). If people are familiar with using a dichotomous key, I send them to Muencher's Keys to Wood Plants (by Edward Cope). There are lots of other books for different groups of plants (such as a new sedge guide called Sedges of Maine that is with hundreds of photographs), but those are the groups that I suspect most of those interested in permaculture would be interested in. I hope that helps some.