Yes, it appears to be a legitimate compilation from the writings of John Wesley. I haven't confirmed all of the many stories in Jennings' book, but I selected several and found them corroborated in other editions of Wesley's journals (which are indeed available online).
- Page 5, "The Woodseats' Demoniac":
- Page 37, "The Michaelmas-Day Demon":
- Page 87, "Whole House Overtaken By The Holy Spirit's Power"
You can use Google books to search for the exact wording of any particular story to confirm its origin and find it in an official print edition. An abridgment of Wesley's journals is also available online.