TL:DR Immoral yes, cheating no.
Firstly, it's not technically cheating. If there are publicly available resources and all your friends have internet access and knew in advance what game they would be playing then you've not done anything that another player could not have done if they chose to.
If you should however depends on your motivation for playing a game. If its to win at all costs, ie in a tournament then by all means research a game and give your self every advantage going. There is not friendship lost in that situation.
If you are playing games for enjoyment of spending time with people and to just play the game then researching strategy before hand make the game less fun for everyone.
It's less fun for you because you don't get the joy of discovering a game for yourself. isn't it fun to look at a games rules and think how you would approach it, not what some internet article tells you to do?
You also make it less fun for every other player. A game is at its most enjoyable when all players are of an equal level, No one enjoys winning or losing by a huge margin. Who wants to spend over half a game knowing they have already won or lost?
To manage this I would suggest the following, firstly don't read strategy guides before playing a game. Would you buy a computer adventure game and then follow a walk through rather than trying to solve it yourself? Would you buy a puzzle book and copy what's in the answers pages on to the puzzle pages?
If you are teaching a game that you're are more experienced at the way to manage this is to share some of your knowledge and warn about pitfalls but still letter players discover their own strategies. When I play games such as World Without End I know all the events off by heart, so I try to warn players and things that might happen. Similarly if I'm playing Carcassonne I know the tile distribution, so If know all 6 Cloister tiles have been player why wouldn't I share that information with my opponent? I want them to have same public knowledge that I have and they can then choose what to do with that information.
Reading your question you say you told your friends AFTER you had read a strategy guide. If I was your friends I would be really annoyed with you as well. I'd be struggling to understand their motivation for doing so and would conclude winning a game meant more to them than actually playing a game and spending time with them. So no you didn't cheat but its was certainly morally dubious.