I did a very short blog post awhile back and listed a bunch of books to explore. I, personally, like reading and getting ideas.
Often, it's your client base that will help you learn what they really appreciate and also, give hints as to what they desire.
When I first started, I went to Goodwill and bought a bunch of REAL silk ties. Smile. Then, just some regular rope. And then you can go to the Domme Depot, wait ... that's Home Depot ... and find other stuff.
So much of "this", though, is mental. It's a mind fuck. After you realize that, just a gentle whisper can cause a man to almost go mad with desire and alternative thoughts.
You can use a fork for sensation play. A spatula,ruler, leather belt, hairbrush for spanking. A scarf to gently cover their eyes. An Hitachi wand to drive them nuts in other ways. OR ... and this is something I just learned ... a disposable vibrating toothbrush to tickle that part of his body between the penis and the anus. Or nipples as well!
Once you get an idea of what you wish to DO, then you can start spending money. But when you're starting, don't waste money on too many toys. See what you have in the house.
And remember in terms of LEVELS of play: "One man's kink is another man's vanilla". This will serve you well on your journey.