Scamp - if you are quoting directly from the book, I think you may be right that the writer is saying that Glenn came up with guitar parts that Felder's fingers couldn't deliver. It makes sense because there are various accounts about times such as ICTYW and TALA when Glenn was able to create a better recorded version than either Felder or Joe, but he didn't perform the part live. I doubt if there are any Eagles songs that any of the guitar players couldn't actually play - it was just a matter of who played the best or preferred version on the record.