Hello, I don't know of other resources nor would I call myself an expert player but here is the basic calculus. City connections are always optimal play because they provide gold and sometimes happiness (Liberty policy or the Exploration Harbor policy for sea connections.) Almost all early connections will be made with Roads, even between cities which are both coastal on the same continent, because the mobility bonus is useful (for Workers and GP as much as military) and it saves early Production on Harbors which are kind of weak in themselves (instead requiring Worker power which is plentiful if you steal many in the early game). But it takes some practice to figure out when a road should be prioritized, because many factors determine its importance, including:
-size of the capital (Bigger capital will improve connection gold from every city, making it always important to grow even without Tradition)
-size of the other city
-length of the road; most road costs will break even with connection gold by the time your Workers are ready to build them
-How badly you need gold in the short-term
-Worker power; the profit from connections will usually not large enough to pull your Workers away from improving tiles while your cities are first growing. If you have an excess of Workers you can start Roads earlier, otherwise you will usually wait until your cities are growing more slowly and there are unworked, improved tiles for them to grow into.
-Mobility. Rougher terrain makes roads better, and you can move units from one side of the empire to the other quickly, meaning you need fewer units overall to feel secure at your borders.
Harbors cost 2gpt maintenance, so your first naval connection will cost 4gpt, and subsequent ones 2gpt. This can lead to nice profit, but it sucks fitting Harbors into the builds of your expansion cities, because they cost many hammers and you can end up delaying them a lot based on always having something better to build. Happiness policies make early Harbors better. Carthage is supreme because her Harbors cost neither gold nor production. In theory, Harbor connections are vulnerable to naval blockades but the AI will never take full advantage of this.
And Harbors are much better than roads once Railroad is unlocked in the late game. Not only do they get the RR production bonus immediately, instead of having to wait for Workers to change your whole road network, they will avoid the increased maintenance of railroads as well. This is probably why you have seen players deleting certain roads. If a sea connection is possible, there will usually come a point a little before RR that building a Harbor and deleting the road can pay off in production and gold. But they are never getting rid of city connections.
