Somewhere in the depths of my bookcases I've a booklet produced by someone several years ago about this route. I'll try and dig it up. (It is for me a local route - bus stop about a minute's walk away!)
The 64 page booklet is called "84 - Bus to St Albans" - written by Tony Billings and published by him in 2003 - ISBN 0 9508803 9 6. It contains numerous b/w photos
It reviews the service, which started in 1912, on the 3rd August, by the London General Omnibus Company. It used their 'B' type buses working out of Cricklewood garage. The 84 was looked upon as more of an 'excursion' route and at first only ran on Sats/Suns and bank Holiday Mondays. The booklet goes on to describe the development of the route and the effect it had on travel in the district.
Regards,
John Webb