While this annual publication is not strictly a trades directory it contains some useful information about the towns in Hertfordshire, and a diary of the main events in the preceding year. As such it is well worth a mention. Microfilm copies are available in the Hemel Hempstead Library and possibly elsewhere in the county. I am not sure when it started publishing but the first reference I have to it is 1849 which contained limited information compared with that available say 20 years later.
If you can add to the information given above tell me.