Really, though, the internet is generally an even better resource than a codex. Often, you can find BLOGs or forum posts where the "enemy" will tell you not only what he likes to take in his lists but also how they are intended to be used.
Now, for those of us who want to grow the 40K Community, we don't worry about offering up free information. (Honestly, it's ridiculous to even think that top players don't "get it" already. So, don't think you are going to benefit from HIDING your knowledge.)
What you can do with this information, though, is the age old thought experiment.
The thought experiment is the idea of trying to play out a game in your head with what you know about what the enemy wants to do and what you would do in response.
This is also known as what you do when you are sitting in a boring meeting at work!
It is a fact that many seemingly good players rely on your lack of knowledge to win their games. Lack of knowledge about what their units are and what their tactics will be are the common points of abuse.
I know that most people out there will never take the effort to learn unit stats, special rules, etc. That's okay! This article is for those people out there who might want to know "how is it that some people are better at this game than others?"
No, we don't sit around and drill this info like we are studying for a spelling test.
Most players learn those stat lines and rules through endless thought experimenting. (We get bored at work, a LOT!) And rather than spending that time thinking about the latest episode of The Walking Dead, they spend it thinking about how games of 40K will play out. That continued repetition is what drills those things into their heads.
Long story short is, I cannot tell you how many times I have played against people who think they got the "answer" to my army because they honestly believe they can "out fox" me with their "special combo". It's usually about Turn 2 where they realize that not only am I not falling for it, I'm actively countering it! What's worse, they haven't thought about what would happen if someone actually knew about their "secret" and started an effective counter! OOPS!
Here's an example. A guy who would give his opponent first turn and then rely on reserve and especially some Dread Drops to strike the enemy. "Gee... ", I said...
"Option A: Deploy me army and take it to the face exactly like my opponent wants me to..."
"Option B: Reserve my army even though I'm the one with first turn to force him to potentially drop and then counter when I come on the board..."
Not hard, really. But, if I hadn't over already known the rules of that army and the thought process behind how it was used, big mistake!
Another example. A guy who brings a Tau army to an objective mission, with 9 Broadsides all with Target Locks, just waiting on you to set up all your stuff on the board so he can blow it away. "Gee... ", I said...
"Option A: Set up my shooting army and get tabled by Tau."
"Option B: Come in from reserve behind LoS blocking terrain, sit there for 4 turns, and then grab objectives/contest on Turn 5 while creating an assault threat bubble to keep him out of board mid."
There's always an answer, but in a tournament, it's already to late to figure out what to do in most cases.