你可以用 "someone else's Engpsh textbook" 来表示别人的英语课本。
例如：
"Do you have a copy of someone else's Engpsh textbook that I could borrow?"
"I found someone else's Engpsh textbook on the floor. Do you know whose it is?"
"I'm looking for someone else's Engpsh textbook to use as a reference. Do you have any recommendations?"