Robson's Commercial Directory of London & the Nine Home Counties of ... Herts ... was published in 1838. I have not seen it so don't know how extensive its coverage was - but is unlikely to have been more extensive than Pigot's Directories of the same period.
There are few countywide directories apart from Kelly's Post Office Directories. Williams' Directory of the Principal Market Towns in Hertfordshire, dated 1850, is only known from one patched up copy.
The 1854 Craven & Co Commercial Directory of Bedfordshire & Hertfordshire clearly takes descriptive material from the 1851 Post Office Directory, but contains more detailed lists of gentry and traders, and a very interesting collection of advertisements.
Thomas Danks published the History, Topography and Directory of Bedfordshire, Huntingdonshire and Hertfordshire in 1863. In many ways the directory information is very similar to Post Office directory - and some of it appears to have been copied from that source.
See LDS Holdings of Hertfordshire Directories
If you can add to the information given above tell me.