I'm in the same boat as you! As a disclaimer, I definitely am still learning ID sketching and have a LONG way to go, but I'll throw in a few beginning sketching tips and let the master ID sketchers on Core correct me where needed!
You're off to a great start! I definitely recognize that paint roller example, so it's good to see that you already picked up some ID sketching books. If you list what you currently have in your library, we can make some recommendations for you.
As you've already started, keep it up with the stack o' 8.5x11 paper. Keep burning through that stack with linework practice of freehand straight lines and ellipses and perspective sketches.
As far as materials go, I'd focus on adding in some more linework options. It's actually pretty amazing how different each one feels and looks on the page. It can't hurt to experiment with a few sheets of each then pick one type (pencil, ballpoint, felt tip) to stick to for a while to give you some consistancy. Here's a few to check out.
Prismacolor Premier Pencil
Prismacolor Verethin Pencil
Steadtler Triplus Fineliner
Bic Cristal Ballpoint
Keep it up!