I realise you probably don't want to spend too much money, but Michael McNally's Teutoburg Forest, AD 9: The destruction of Varus and his legions is a damn good read. It's one of Osprey Publishing's books, so there's plenty of maps and very good context and narrative. Be warned though, my copy cost £15 (U.S. $21.95).

What you really need is a copy of Pliny's "History of the Germanic Wars". Unfortunately, no copy is known to exist. Your local library may have access to JSTOR and other databases for free, or if you have a university library nearby. That's a good place to start.

My thesis was on Roman operations in Germania during Augustus' reign, so I may be able to answer any questions.

Some good sources, other than those already mentioned:

Delbrück, Hans. History of the Art of War, Volume II: The Barbarian Invasions. - Dated, but highly influential in it's time.

Murdoch, Adrian. Rome's Greatest Defeat: Massacre in the Teutoburg Forest.

Rost, Achim and Susanne Wilbers-Rost. “Weapons at the Battlefield of Kalkriese.” Galdius XXX (2010), 117-136.

Wells, Peter S.. The Barbarians Speak: How the Conquered Peoples Shaped Roman Europe.

Wilbers-Rost, Susanne, “Total Roman Defeat at the Battle of Varus (9 AD).” in Fields of Conflict

A few things to think about:

The Roman army in AD 9 was in bad shape. The rebellion in Illyricum had sapped Rome's resources severely. Augustus was forced to raise new taxes, and start forced conscription to fight it. Varus' legions may have been understrength because of this. A Roman legion of the period was 4800 men on paper (There is no evidence of the first cohort's double-strength centuries this early). Varus had detachments all over the area, most of whom were killed away from Kalkriese. He had left a strong force at 'Aliso' (probably Haltern), probably cohort sized, and perhaps another cohort at Xanten. Those would also have included any too sick to march to the summer camp. The 6 cohorts of light infantry and 3 alae of cavalry would have had significant numbers of Germans, who probably either switched sides or simply slipped away. Furthermore, a number of survivors slipped away and made it to Aliso. Thus the casualty numbers at Teutoburg may not have numbered more that 10-12000.

The survivors are a mystery. Kalkriese is a long way from Haltern (if the fort called Aliso is Haltern). Either they were the remains of Numinius Vala's cavalry that managed to break out, or a group large enough to defend themselves broke out of the trap. It's hard to imagine that scattered stragglers could make it to Haltern without being ridden down by the Germanic cavalry.

The Roman problems in dominating Germania were largely logistic. The Germanic tribes weren't urbanized like the Gauls, and so had little surplus agriculture to feed the occupying legions. Because Roman supplies moved by riverboat, they could easily dominate the Rhine, Lippe, and Main River valleys. Beyond that was problematic. Drusus had built a canal from the Rhine through what is now the IJselmeer to the Wadden Sea, but supplies still had to run a gauntlet from there to the Wesser or the Elbe. The North Sea storms made winter resupply all but impossible. In the spring, forward supply depots could be established on those rivers for the summer, but the Romans were forced to retreat to the Rhine/Lippe region. This always gave the Germani a breather.

There is a strong argument that the battle itself was not the end of Augustus' ambition in Germania. Varus' two legions that remained on the Rhine were reinforced with six more legions. Germanicus used those legions to raid in to Germania shortly before Augustus' death in AD 14, and laid waste to much of the area between the Lippe and Weser, including defeating a very large force in a pitched battle and recovering two of the lost eagles. For the most part, the tribes hostile to Rome appear to have withdrawn beyond the Elbe. Augustus' deathbed command to keep to the current borders is a bit too convenient for Tiberius. Tiberius had good reasons not to continue to go into Germania. Aside from the poor condition of the army, Tiberius didn't trust his nephew with eight legions that might march on Rome. The Rhine command would be broken up into two provinces with four legions each.

Varus' summer camp location, and the route of march, are still unknown. I favor Barkhausen for the summer camp, and the route of march along the north slope of the Wiehengeberge.

The book The Battle That Stopped Rome: Emperor Augustus, Arminius, and the Slaughter of the Legions in the Teutoburg Forest by Peter S. Wells is underrated.