Finding good, detailed field guides to plants, animals and birds in Botswana is relatively easy within the country. There are now very comprehensive guides on the flora and fauna of southern Africa – which usually cover Botswana – and several guides dedicated just to Botswana.

Top of your list should be Veronica Roodt's Trees & Shrubs, even if you're not that interested in trees or shrubs. I've always used Newman's Birds of Southern Africa, by Kenneth Newman, although the same author has a specific Birds of Botswana in print which may be more appropriate if you're confining your visit to Botswana.

Finally, if you have room and lots of time, then Richard Estes' Safari Companion is a great general book on mammalian behaviour.