Which sources can I use to study botany and horticulture?
This book could be a good start: Botany by Mauseth. However if you don't have any knowledge about biology I suggest you to get starting with the basics of cellular biology. A good book could be "The cell" by Cooper, you can find it in electronic version.  There are many books about cellular biology but I recommended this because it is complete and detailed without being too long compared with others.