A espioneage economy works well if the real economy is down the toilet due to too much conquest.
This is bacause the really strong espionage infrastructure comes late in the game - so it basicly does not matter, how badly behind your infrasturcture is in medieval Age. As long as get Constitution not too long after the AI does, you are fine.
Get yourself to CoL somehow, if not there yet. Build Courthouses, start running spy Specialists in them. Start building up espionage Points - a little against everyone, and lots agains the Civ you want to steal from... The closest civ that has a tech on you in the beginning, later, the tech leader.
You only really need Alphabet, CoL and Optics - if there is no one to steal nearbye. Usually you can steal Optics and some Backfill techs from a neigbour, then go after more advanced civs on the other Lands... As soon as those 3 needed techs are done, forgot your Science slider - drop it too 0. Run some Spy Slider if you can, just money if you need - you will do fine with just the Spy Specialists, if has to. If you happen to pop a Great Spy (if you cant make on now is ok, but try not to get too many other GP's - you can pop some GSpys after you have Jails), Build Scotland Yard(s).
Dont need to ever build those expensive Tech buildings - Library, Uni, Obs. Just direct your Hammeers towars increasing income (Markets) and military. Of course you also need a good load of spy's and 5 to 10 Caravels, if stealing overseas. A couple Missionaries are good too, as stelaing from a city that has your religion is cheaper. So spread first. Drop Spys in the city you choosesen (Small City, as close to you as possible, with your religion) and let them sleep there for 5 turns for maximum discount. Allways take care of resupplieng spys - they will get cought from time to time...
Beeline (Stealing not self teching) the Techs that lead to Constitution, save up some EP's to steal Constitution ASAP. You will see what the AI is Teching soon, so you can choose who to direct your EP's against. Once you have Constitution, Build Jails after that you basicly can steal what you want. By now you should have some matured cottages, to give you money and also be able to take the Espionage Slider up to give you a lot more EP's...
I recovered out of some really bad holes this way - like 15 techs behind, beeing early medieval, when the first AI's get Liberalism... - it works really nice with large empires, as every city can contribute via Buildings, regardles the slider. Lends itself more for Military games: Steling from the Tech leader, using the Techs to Bully the more Backward civs. But if you have nice production, you can go Space as well.