Guide by Big Al from Oz at http://warbookforums.com/smf/index.php?topic=207.0

This idea is very simple and effective, but may cause surprise.

There are two main sources of gold -- 1. buildings (mainly mines) and 2. alchemy.

Alchemy is not suited to maintaining troops. On the other hand, building income is probably best devoted 100% to maintaining troops. Do you want to withstand attack and have the power to hit back and take more land? Well, we clearly want the largest possible army to do those jobs. That involves using all available building income for troop maintanence, and doing so most efficiently and flexibly. We will discuss those details in another post. That aspect of WarBook is intensely dependent on particular hero type.

Alchemy provides the majority of income, unless you are maintaining less than about 2/3 of your maximum standing army, because alchemy is based on total income, not income after maintanence. If you are maintaining maximum standing army, alchemy is 100% of income. This is why magicians are the highest income class. They can regenerate mana twice as fast as other hero types. More importantly, this is why you should aim to reach at least 60 mana as soon as possible. It maximizes income and hence maximizes growth. The cost of alchemy increases after 60 mana, so that three hours worth of mana buys one alchemy spell (or two spells per 3 hours for magicians).

Good as far as it goes
At first, I used to try to keep a nice income level, so I had plenty of cash to do whatever was needed. Cash is nice and flexible. I kept getting hit, no probs, that's the game. Smart me has plenty of cash to replace troops and buy land back.
Then I sat down and thought about it, build my army right up and use mana for my cash. People stopped hitting me. My army was now too strong (or at least stronger than others at my acreage). By having less income I got more income, because I didn't need to keep rebuilding!