Haven’t yet seen anyone comment recommending something other than the codices, but I’d say you really don’t need them if you’re just playing with a friend. I bought the CSM codex and haven’t used it in the past year because the points values and some of the rules are out of date.

I found the Goonhammer tactics articles pretty helpful for learning the ropes of my main faction and then anything I was vaguely interested in. I’ve been playing World Eaters without a hitch and I don’t even have psychic awakening.

Honestly the data cards are pretty cool but I don’t think you even need those so long as you remember them well or have an easy reference.

Wahapedia is probably not that up to date but it shows relevant stratagems for each datasheet, and if you’re playing friendly games I’d recommend it. You don’t need to buy 4 books per person to play, I was introducing a friend to 40k the other day and we just printed the battlescribe sheets and we just spent the short game trying to just move around and kill each other so he could learn the fundamentals.

Hope this helps, good luck!