It hasn't come up in my campaign, and the other night Jong asked me about it. And I'm too lazy to try looking through some of the less well-organized sections of the rules to try to find a rule I suspect does not exist.

So first, are there rules about fighting on horseback in the main book?
Are there any in any other book?

If so, what's the gist of them?

RPGPundit

There isn't really a specific 'mounted combat' section in the main book it's just tied into the general difficulty section.  A mounted attacker vs a foot soldier gets a +10% advantage for having a height advantage and the grounded attacker gets a -10% penalty for attacking the rider.  That's basically all that's covered.

For OUR group it hasn't come up a lot either but we've added these rules in.  A mounted rider may substitute his mounts strength in place of his own for the first attack and any charge attack.  A charge attack counts as having the impact - and slow qualities even if that weapon normally doesn't.  If the weapon normally has impact then it gets an additional +1.  

That's pretty much it for our hard coded house rules.  Other things we leave to the situation.

How do you handle mounts attacking enemies? Do they take an action from the rider to guide? Do you let the rider AND the mount attack?

We went for simplicity rather then strict authenticity as that seems closer to the heart of Warhammer.

So a rider can direct a mount to attack as the riders full action using the riders WS or the Mounts whichever is better, but the mounts Strength and attacks score.  A rider and a mount can not both attack in the same round there is to great a chance that the rider will hit an actively kicking/biting mount.  Plus the rider would be more concerned with hanging onto a bucking or rearing mount.

Full action... So no parrying or dodging?

Oh yeah that

Full action allows a dodge - but not the parry.  Plus if you are using the mount to attack that would require both hands on the reins to control it.  Shield bonus counts for defense.  

We disallow parrying with an offhand weapon on horse back because we simply couldn't envision someone attack with both hands to the same side of the horse with any real ability.  Especially if the mount is attacking - so no free parry for an offhand weapon.

However if the rider is attacking then the half action parrying rules apply.  Though not on a charge.

Umm think that about covers it.  Again I'm doing this primarily from memory as my notes are at home.  So far in the 8 or so months that we've been playing I think we've only had mounted combat come up twice.  Once was pretty detailed and required us (well mostly me) coming up with rules that we all agreed on.  The second combat the modified charge rules pretty much ensured that there was no second round of that combat  

Oh that does remind me one more fun rule.  An opponent that can brace a weapon against a charge (wins initiative).  They may use the horses strength instead of their own.  We only allow pole arms, spears and lances to be braced and they already have the impact quality so nothing more in those terms needed to be done.