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As I think you're already considering (given the chariot question), the best book I think mainly depends on what type of forces/troops you want to use. Quite a few Viking Age factions will have no or merely light cavalry, while more melee oriented cav is more common in the Crusades era (almost as if it's based on history!). I can't currenlty provide a full list of all options for all factions, but that seems a practical consideration. At a more detailed level, you may have specific forces in mind, thus perhaps warranting a search for somewhat similar playstyles. Anglo-Saxons for example have always been slightly more of a horde factions (large groups of levy), which may fit well for goblins or the likes. Others are better with elites (Elves?), play defensively (Dwarves?), etc.
There's the possibility of using a chariot in A&A, a mount for the warlord (more status than fighting, but rules nonetheless). If relevant, I think A&A also has the only war machine to date (Roman ballista).
I think multibase figures (which is sort of similar to using a troll - fixed area representing multiple troops) aren't perfect for SAGA, but will work much better in V2.0 than previously. In the past, the units walked fairly freely across the board, taking different formations (just stay within 2" of another bloke in the unit). Now, units have to be tighter, and automatically all fight in combats, making multibasing less of an issue. As such, I think you can get away with it now - whether a base happens to have 2 guys on it, or represents 4 figures in a single troll, just adjust hitpoints and generated attacks accordingly (losing guys or the trolls getting injured). Other than that, if it thematically makes sense, the easiest thing is to have a monster as the warlord: they very much are monsters in combat, are supposed to be on large bases and make a nice centrepiece.
One of my first games of SAGA (V1.0) saw my Vikings face an evil armoured giant (Sauron) and his horde of Orcs (warriors) and Uruk-hai (hearthguard) - counted as Anglo Danes at the time. (Naturally, my warlord heroically duelled the monstrous adversary and gained some shiney new jewelry for his troubles. No idea what eventually came of him or his loot made of precious metals...)