Can anyone give me background on Erasmus Haarlock WITHOUT telling to look in/buy one the Dark Heresy books?

Not easily, since the background of Erasmus Haarlock makes up a lot of critical plot points in the Haarlock trilogy. Posting them here would probably be a great way to make FFG banned of it.

The background on the Haarlock line is somewhat covered in the unofficial timelines of Dark Heresy, Rogue Trader and Deathwatch however, so you can give those a read for the broad strokes. The Haarlock line rose to prominence during the Age of Apostasy when the Free Trader Mordecai Haarlock was instrumental in freeing the Jericho Sector from the tyranny of the minions of Goge Vandire. The Jericho Sector didn't turn out that well, but the Haarlock warrant was one of the most powerful that the High Lords of Terra could grant, and gave his family vast, sweeping powers. They grew in strength and control over the millennia culminating in the Calixis Sector basically owing their prosperity to the Haarlock family, and their influence could be felt over the sector.

Then Erasumus Haarlock, recent Warrant holder of their dynasty died under mysterious circumstances - which was perfectly normal for Rogue Traders - and the Warrant passed to his next of kin and everything seemed fine. Then the Haarlock family all died, and it was said to be the curse of Erasmus Haarlock coming back from the grave to purge his family, which was done. All known claimants to the Haarlock warrant died, and anyone who tried to come forward to claim it also died. As an apparent side-consequence of this, the Navigator House Nostromo was also devastate by these events as they had invested nearly their entire house into an alliance with the Haarlocks, and the death of the Haarlock family and fleet led to the destruction of that Navigator House.

At the start of Dark Heresy, the Haarlock line has been declared officially extinct, and their assets are being sold off.

In the DH forum they had a huge debate over it. the link can be found here. It's a nice read. About 24ish pages worth of information. Happy hunting.

I COULD just tell you the information from the DH books...but that seems a little pointless and wasteful when you could get it just as easily by buying them .

But in all seriousness, it depends what LEVEL of background you want. Erathia has got the layman's knowledge to a decent extent - though with a couple of innaccuracies: Erasmus initially just disappeared during the Haarlock line's bizarre succession ritual (all potential candidates fight each other to determine who gets the warrant, you can surrender or die at any time, after everyone else has surrendered or died official representatives from Terra say you're the one who gets the warrant) and came back after 10 years out in the Expanse, after which the Haarlocks began dying in droves, reportedly at Erasmus' hands. He was named the RT of the Haarlock line, and continued to hunt down various bastards and offshoots as well as (according to some dark rumours) do some more dubious things that made the previous excesses of his line look Inquisition approved. He was last seen in M41.703 and is thought to be dead.

A more DETAILED version would know that he consorted with daemons great and small, was obsessed with clockwork and the number 13, was greatly feared by a number of xenos races (including the motherfucking Slaugth) and had pimped out his vessel (the frigate The Spear of Destiny ) with some serious warp **** (there's a part in a DH adventure where it suddenly shows up - like literally, just suddenly IS THERE NO WARP DRIVES NO NOTHING and wipes out a couple of ships in two volleys of devastating purple bull). They might even mention that he was apparently nothing special before the death of his wife and daughter at the hands of his sorcerous cousin, and that this is what drove him into the expanse.

Also, fun fact - his to the x Great Grandfather, Solomon Haarlock, is the man who discovered the Calyx Expanse. This became the Calixis Sector. The house symbol is a black sun and golden spider. The Calixis Sector is famed for the strange black sun known as the Tyrant Star.