Is there an equivalent to "The Man in Seat Sixty-One" for bus travel?

This doesn't exactly answer your question, but I don't think you'll find a single, centralised resource.

There's, of course, a reason behind why it's humanly possible to put together a resource like Seat 61. In many countries, trains are either nationalised (most countries) - either fully or like in the UK where the infrastructure is nationalised but the train operations aren't. Regardless of how the infrastructure is operated, train services are often either run by a single company / public body or an oligopoly of companies because of the capital involved and the limited nature of rail network 'resource'.

Bus companies in most countries on the other hand are more loosely regulated than trains or even other sectors such as airlines. These may still be a handful in Europe and North America, but the moment you get to Asia things get more complex as you'll literally find dozens of companies operating. Additionally, unlike train and airline reservation and scheduling systems which are largely computerised, bus services in many countries aren't at all making it harder to put together a centralised resource.

You'd be better off asking for bus schedule and information resources for individual countries or regions you're interested in.

EDIT: I recently came across this website called The Bus Station. It does not contain in-depth articles and guides as Seat61 does but for many countries it does have website / bus operator listings. Not the nicest site on your eyes though! Better than nothing I'd say if you can still at least look up bus schedules online. One thing to note could be that in parts of the world, bus operators offering online booking / schedules might be costlier ones and you may as well find cheaper operators once you're in the country.

A new website has appeared on the market recently called BusBud. It lists schedules and lets you book tickets in many countries around the world. They probably don't cover "hole in the wall" kind of operations, but that's the best I have seen.

Disclaimer: I'm not affiliated with BusBud, but one of the developers is active on this site.

Since another answer has been posted mentioning BusBud, I would like to add two other resources I use to find bus routes. I have tested it on two little known bus lines between the Basilicata region of Italy and Rome and Fiumicino airport of Rome and both outperformed.

• BusRadar is a bus travel search engine, active only in Europe as of now. At least in some countries (Germany for sure) they also offer alternative trains and Blablacars as alternative connections.
• Rome2Rio is searching for connections using pretty much any means of transport and thus not limited to bus connections but you are free to look at buses only. The site coverage is worldwide, but you can not initially specify your dates of travel.

Of course none of these sites is complete in capturing all lines, if you are looking for a specific route it also helps to google or look at the get-there section of your guidebook (LP is very good). Also none of the sites is in any way comparable to the Man in Seat Sixty-One.