Lynx Ediciones of Barcelona publish a book in English called "
Where to Watch Birds in Doņana", which might be of interest. Otherwise the Helm "
Where to Watch Birds in Southern Spain" is pretty useful as well. Note that this link is to the forthcoming 3rd edition: there are also earlier editions for sale as well.
Another useful resource for maps & guides to wildlife in spain is
La Tienda Verde, a delightful pair of shops in Madrid, but also with a good on-line catalogue.
If you can read Spanish there is also an
Everest guide to Doņana. Everest are a spanish publisher of colourful guides to the spanish regions, along Michelin lines. I found their guide to Extramadura really useful because it had listings of accommodation even in the smaller villages: a real plus over the limited listings in the english language guides.
I haven't found anything directly on Plants, Insects etc. for the area.
HTH,
poschiavanus