So the series I read was The Warlord Chronicles by Bernard Cornwell. The author claims that soldiers will often balk at the idea of charging a well formed shield wall, sometimes standing for hours until they get up the courage or receive the appropriate coaxing from their superiors. The second claim is that horses will not charge a shield wall. He says that often a cavalry charge will make the shield wall break due to the soldiers fear, but if the wall holds the cavalry will veer off. How accurate are these claims? Are there any accounts of battles that illustrate this?
I've discussed how infantry warfare in the Early and High Middle Ages possibly worked previously. With regards to Cornwell's interpretation, I do mostly disagree with his view of drunken courage and extreme reluctance of men to approach to within spear range of their opponents.
Regarding horses and shieldwalls, it's absolutely true that a charge of horsemen can cause inexperienced or poorly disciplined infantry, or infantry with poor morale, to break even before the charge. For exmaple William Tomkinson, when discussing his experiences of the Peninsula Campaign, wrote of how you could tell that infantry was going to break by the shuffling of their feet and the wavering of their muskets. Winston Churchill, in describing the events of the Battle of Omdurman, observed that either the infantry broke before the charge arrived or the cavalry charge broke before contact, thus making the charge he was involved in exceptional.
The question of whether or not horses will charge a shield wall is less about the horses and more about the men. While Bernard Cornwell is a faithful adherent of John Keegan and his belief that horses are too smart to charge into infantry, Keegan did not make anywhere near a comprehensive survey of 19th century battle accounts and drew his conclusions solely from the accounts of infantryman. Napoleonic generals, such as Bismarck and Marmont, largely put the failure of cavalry to charge infantry squares down to the fear of the rider, with Marmont adding that the way in which some cavalry units trained - pulling their horses up short of infantry squares - actually conditioned the horses to baulk in battle. Tomkinson's diary suggests a similar role in the rider deciding to avoid impact, and Captain Louis Nolan, writing fifty years later, has three pages listing battles in which cavalry physically broke infantry squares rather than doing so psychologically, and has several other anecdotes showing the same thing.
In short, I believe that horses would absolutely charge into a shield wall, but their riders would try and avoid it under most circumstances because they're not suicidal.