Online
My constant source of information has been this online Virtual Herbarium of the Westerm Mediterranea and its section devoted to the Balearic Islands
http://herbarivirtual.uib.es/cas-uib/index.html
helped by this newly published book on the flora of Menorca of which a pdf version has generously been put on line.
http://lifereneix.cime.es/WebEditor/Pagines/file/Plantes%20de%20Menorca%20baixa.pdf
Always useful of course are:
http://www.floracatalana.net/visual?page=1
For up to date information of botanical names there is always:
And there is this wonderful collection of photographs by Jose Luis Quiles
Other sites have been used, for example to look for German names of plants an extra information on specific plants.
Books
As my interest in plants began long before the Internet, I have a large selection of books of which the following
have also been useful:
Also for up to date information on plant classification.
Mabberley's Plant-Book Third Edition A new edition has just been published but am still wondering whether I should buy it.
Old friends -
Oleg Polunin's books - Flowers of Europe - Flowers of South-West Europe
Wild Flowers of Majorca Minorca Ibiza by Elspeth Becket