I've only recently started playing D&D, having previously played an MMORPG for a year or two.
One thing I've noticed moving to D&D is the lack of strategy guides. I've found plenty of DM guides for encounter ideas or how to deal with groups that always use the same annoying tactic, but nothing about general tactics.
The number of different situations are practically infinite, so I'm not intending to cover all of them. Instead I'm planning to cover broad situations and how you might handle them - as a result it's not D&D specific, it should apply to any RPG.
A situation example is fighting through a bottleneck such as a door between rooms. It might be easy to defend against melee, but what if they're all ranged or controllers? What if you drop back and let them take the bottleneck? Even simple situations can be complicated, so I'll just cover what I can.
My expectation is that this guide will be as useful for DMs as for players.
Also, if you have a specific situation you want analysing to get some ideas of how to deal with it, let me know and maybe I'll cover it in a post.