For horticulture I recommend using phone apps with your camera to explore the open areas around you and reading about the plants you discover. This will get you started so that you know what to look for and then in a local library you will know what more specific books to request by research topic.
For philosophy I recommend starting with the classics. Most of this is public domain and online. Examples: Aristotle, Plato, Mills, Descartes, Kant, Adam Smith. Then I recommend moving onto modern philosophy, such as philosophy of language, religion, and ethics. You could even find compilation books of philosophy by subject which then provide a single starting point of excellent research sources plus bibliography at the back that directly references more things to read about.
Start reading. Then once you have gotten through some of this material you will become better aware of other subjects and materials to research and how to find them.
Especially looking at old recipes will cross a number of disciplines, including maths, chemistry, linguistics and library science.
OpenStax has textbooks available for no money. At least as reference, maybe not fun?
If you have access to a public lending library, consider asking a librarian for help with this, too.