If the idea is to have battles with a medieval feel, then the choice lies on Epitoma rei militaris by P. Vegati Renati (now better known as Vegetius).
While Rome produced many excellent military treatises, this one was the most popular throughout the Middles Ages in Europe. His work was studied, rewritten and adapted to the new techniques as they appeared, and quite a few princes (at least in the Iberian Peninsula) studied it thoroughly.
Now that you've read Vegetius, choose the area of Europe that you want to use as basis for your fantasy work. This choice is important because Northern Europe moved to heavy armor for both knight and horse earlier than the Iberian Kingdoms, since the latter favoured fast horses. Also, England was famous for the longbow, while the Iberian Kingdoms favoured the crossbow for its accuracy (spiced with some poison, obviously). I don't know the particularities of German and Italian warfare of the time, but take a look at them before choosing what matches your fantasy world.
Once you've chosen your area, look for treatises that locals of the time produced. Even if they were 'translations' of Vegetius, they'll have new sections to mention the techniques developed since. In the Iberian Peninsula, there is quite a history of military treatises in the 11th-15th centuries. Some were copies, some were more original. For example, King Alfonso X of Castille wrote the famous Siete Partidas in the 12th century, which includes a veritable treatise on warfare in its second section. In the 14th century, Don Juan Manuel wrote another treatise on War, but the influence of Vegetius is clear throughout his work.
After you've gone through the work of the time period, look to the present. Find a Military Encyclopedia or similar and study all the big battles of the time period you're after. Don't limit yourself to the area you've chosen, but look at others for it will give you an insight into, eg, why the English relied so much on their longbow, while the Iberian Kings prized the crossbow and forced the infantry to practise its use regularly.
Battles where the smaller army beat the bigger one will be of particular interest, but reading about the bigger armies' victories or the evolution of well balanced opponents will help you understand how the smaller armies could take advantage of traditional moves.