The combat book is found in a recruit's bunk during the Legacy of Seergazequest. It tells the player about flails and that the Vyrewatch might be vulnerable to such a weapon. The only unique option available is to read it which informs the player,
You begin to read the book on combat, it seems quite worn. You skip past spears on page 70 and hammers on page 68, but you enjoy reading a little about ranged weapons on page 74.
The entry on flails at page 73 is quite brief - taking note of the importance of correct posture on page 76 and suitable armour options on page 78, you realise that the flail is probably the weapon you're looking for.
Reading briefly, you realise the flail is a difficult weapon to master. Many quotes from eminent warriors attest to the weapon's 'strange handling' and 'unwieldy manner of attack'.