Out of your answers it would be best to say "Can you share your textbook with me?" and
"Can we share your textbook?"
Though normally people wold say some thing like "I forgot my textbook, would you mind sharing yours with me?"
It sounds better and much more polite. It's like the debate with "Can I got to the washroom?" and "May I go to the washroom?"
Yes, you are capable of going to the washroom, but right now you are asking for permission to go to the washroom. So naturally you'd pick the second answer.
I hope this makes sense.