The thing with source books of this nature is that your usually just paying for a whole lot of fluff, and not a whole lot of crunch. The Slayers Guide to Orcs may be an adequate source, but as an owner of the Slayers Guide to Hobgoblins, I doubt its going to provide much that will impress you.
There are a few things out there that can make very good indirect sources if you want some more crunch to go with the fluff. I suggest taking a look at Traps and Treachery, and the Book of Vile Darkness. You may also want to take a look around for prestige classes involving Grummsh.
And now, for my own thoughts about how to run Orcs, and how they stack up against other humanoid types (Goblins, Kobolds, Hobgoblins).
Assets
Good physical power: They are simply stronger then any other single HD humanoids.
Good shamans: Orcs are alot more iconic than the other humanoids. I would translate that by having them benefit from the intervention of their gods more often than Hobgoblins. More preist / cleric types, and more powerful preist / cleric types. Its not that they are more devout. Its that they tend to better embody their gods ideals.
Good Connections: If your an evil warlord and you need warm bodies to swing axes, you think Orc. If your an evil wizard and want some loyal but malleable minions to dominate, you think Orc. If your an Ogre Cheiftan and want to fill out your numbers, you think Orc. If your looking for mercenaries who will kill for cash on the cheap, you think Orc. There is essentially no evil race or leader that Orcs would not work for, and they are in demand. Goblins are too weak, and Hobgoblins too expensive and arrogant / ambitious.
Disadvantages
Stupidity: Compared to Hobgoblins, and probably goblins, they are dumb. This manifests in being more easily tricked, and in having poorer tactical thinking. The height of orcish strategy is typically to throw more Orcs at it until it falls over. Unfortunately, this is a strategy they are usually well equpied to pursue.
Poor Equipment: There is a reason that there are no fameous Orcish Blacksmiths. Their weapons are going to be either poorly made or in poor repair if they are primarily metal. Variants of leather and hide armor are going to cover standard issue. And they probably arent using longbows. When they have better gear, its either made by slaves, stolen, or aquired by trade.
Treachary and Infighting: Being chaotic, the strongest rise to the top. The strongest will try to stay at the top by arranging for their rivals to die. The weakest will get around this by hanging their rivals out to die as the opportunities arise. So Orc tribes are likely to have say, a 10th level cheif, with most other classed Orcs being 3rd level or lower. A Hobgoblin tribe, by comparison, is more likely to have say, an 7th or 8th level cheif with a number of 5th and 6th sub cheifs.
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