Short answers, pulled from the story.
A royal entry was a formal ceremony in which a ruler or their representative processed into a city, was greeted by civic authorities, and was paid homage, followed by a feast and celebrations. The entry originated as a gesture of loyalty by the city to the ruler, with roots in the Roman adventus, and served to publicly confirm both the ruler's authority and the city's rights and privileges.
Until the mid-14th century, royal entries were relatively simple affairs in which city authorities met the ruler outside the walls and conducted him through decorated streets. From the 14th century onward, with the court of Burgundy in the lead, guilds began incorporating staged allegorical tableaux into the ceremonies, and the occasions grew increasingly elaborate through the 15th and 16th centuries.
Jan van Eyck, Leonardo da Vinci, Albrecht Durer, Holbein, Andrea del Sarto, Tintoretto, Veronese, and Rubens all worked on entry decorations. For court artists like Inigo Jones and Jacques Bellange, designing entries appears to have been their major occupation. Composers including Lassus, Monteverdi, and John Dowland, and writers such as Tasso, Ronsard, and Ben Jonson also contributed.
Alfonso V of Aragon's triumphal entry into Naples in 1443 is considered the earliest triumphal entry in the classical Roman manner in Europe. Alfonso entered seated on a triumphal car under a baldachin; the event was commemorated in a bas-relief on the permanent triumphal arch he built at the Castel Nuovo the same year. An account by Antonio Beccadelli circulated widely and set iconographic models for later entries.
A festival book is a written account of a royal entry or similar festivity, ranging from short pamphlets to lavish illustrated volumes with fold-out panoramas. They are not always reliable; some were compiled from plans before the event and others from fading memories afterward, and Roy Strong described them as idealizations of events, often distant from the reality experienced by onlookers. One Habsburg entry was nearly cancelled by torrential rain, but its festival book shows it as planned.
Royal entries declined in the 17th century for several reasons: rulers became cautious about appearing in slow-moving, publicly announced processions following the assassinations of Henry III and Henry IV of France and the spread of firearms; the cultural atmosphere of Protestantism was less hospitable to the tradition; and a trend led by Medici Florence transferred royal festivities into the private court setting. Louis XIV stopped French royal progresses entirely for over fifty years after taking the throne.