The ABC was invaluable in its day and a good library would subscribe. (I have a shelf full of obsolete editions bought at £1 or £2 per issue.) The problem is that codeshares and flights operating on odd days made it ever more difficult to read and it only went a month or so ahead.
Online timetables timetables can be a pain because you have to input a date. Sometimes you want to know if a flight operates throughout a timetable period and that can be difficult to find. You have to know which airlines operate a route which requires a level of knowledge (and knowing about alliances can help). Not every flight is daily which can cause problems. Some airlines such as QF give the choice of a daily and weekly timetable which is helpful depending on whether you want to fly from SYD - MEL or long haul to a station with an infrequent service. Others might like to.
I did round the world trips in 1990 & 1995. If I was doing the same today I might struggle. I have access to the OAG Cargo Guide which gives widebody passenger flights, websites like FR24 & internet message boards. But where does someone who is not interested in aviation start?