The Parks and Gardens of West Hertfordshire is one of the excellent books published by the Hertfordshire Gardens Trust. It is well illustrated, with many garden plans, some dating back to the eighteenth century. Over 40 West Herts parks and gardens are mentions, in some cases in considerable detail. I was most impressed with the information on Tring Park, where I was walking only a few days ago.
If your ancestors were associated with one of the larger houses in the area, perhaps even as a gardener, this book could be of particular interest, and by getting a copy from the Trust you would be helping the excellent research into historic gardens which they are doing.