Quoted for truth

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No, seriously, read the CIV Illustrated series. It doesn't matter that the stuff is good for Deity-level, those guides explain the most important things like city placement, distribution of wonders and especially economy and as a grand super-issue that you can use as a reference book you can take CIV Illustrated #1 which gives you all infos you will ever want to know about any specific leaders in CIV, it'll help you a lot with diplomacy. Once you have understood those basics (and those guides can really be read in a few hours) you're on Emperor or Immortal. The only thing that's really missing is a guide on the several approaches of war like Axes, Elepult, Horse-Archers, Cuirrassiers etc. but you either find a lot of info on this when searching this forum or the videos from
Absolute Zero (name on youtube chris +some numbers) or people will also tell you something about those approaches if you ask them. It's basically the only guide we weren't able to finish because the war-topic is just too huge for one guide and my time has become more rare unfortunately.