Botany

You can start learning about plants by making use of local field guides. In the EU we have Collin's guides (Tree Guide, Wild Flower). I can also recommend "Identification of Trees and Shrubs in Winter Using Buds and Twigs" by Bernd Schulz which has beautiful illustrations of the details of the twigs and buds of plants during winter, and so it is great for learning to identify trees when they lack leaves. This book is also EU-centered. If you would like a technical book to get really into evolution and taxonomy of plants, I can recommend Plant Systematics by Michael G. Simpson, which is quite good! This one covers plants from around the entire world, but it is not light reading.

For plant nutrition, I read "Soil Science for Gardeners" by Robert Pavlis before I bought the more technical plant nutrition one. Robert Pavlis is the author of a website about gardening myths (https://www.gardenmyths.com/) and he has some books about compost and plant science. I have only read the Soil Science one and it is good. It covers the structure of different types of soil, how nutrients stick to and are release from soil grains as a function of factors such as acidity, the structure of roots, etc... If like this book but feel like you would like to know the details much more in-depth, you can then get Mineral Nutrition of Higher Plants by Horst Marschner, which is a technical text that goes into specifics about the types of nutritients, the concentrations that you can expect under different conditions, their transport into roots and through the plant, metabolism, etc... This book requires a good understanding of chemistry and it is dense, with lots of tables, figures, and data.