I'm not completely certain what the question is, so I hope I end up answering it.
In mage wars, you construct your spellbook ahead of time. That means spellbook construction is done outside of regular playing of the game, and before you ever sit down at the table with your opponent. The base game has enough spells to build 2 decent books with, or you could even build all 4 of the preconstructed books suggested in the rules. Most competitive players will suggest each person have their own, or even multiple, copies of the game but if you're just playing casually this is not necessary.
Now, if you do want to both build books from a shared pool of cards, you are correct that your opponent may get an idea what you're putting into your book. This does not hinder game play, though it does change it somewhat. Instead of building your book and planning your strategies around every possible scenario, you'll be tailoring your game play specifically to your opponent. This can create a sort of arms race as each of you customizes your book to better handle each other. Although this kind of gameplay might not be indicative of a "normal" environment, it is nonetheless fun and can create very exciting matches.
There are multiple fan suggestions for how to draft cards from a single pool if you want ideas other than piling the cards on the table. Search around on these forums as well as bgg.
Different from building your spellbooks though, during the actual game you will be planning your spells in secret. Even if your opponent knows what spells you've put into your book, they will never know what spells you are planning. Most good books contain 2 or 3 strategies they can follow, along with multiple fringe options, which gives you lots of room to surprise and outmaneuver each other.