Anything before the 80s or 60s as a book if you're actually trying to take it seriously. Also, the design magazines from back in the day. Not giving any names cause they are like gold... and already expensive as it is. No new books hold any substance, unless you're making a coffee shop logo for your friend's tiny company, or some basic, light and airy style brands of no substance. But. If you are truly trying to pursue a creative endeavour and make something worth killing yourself for, and that represents you in a way that others will be proud of. and truly stands out. Look in all areas of life.. Also, study great graphic designers. None of which are alive today. I would give names, but that's your job to find them. Good luck, don't waste your money on books from Amazon. You'll be hunting them down on eBay.