Well, the first houses a statue of King George IV on horseback. George, for those like your author who are a little patchy on royal history, was the son of the mad King George III, and was king from 1820 until his death in 1830. He also served as Prince Regent for a further nine years before 1820 during his father's madness.
The second plinth carries a statue of Sir Henry Havelock, a 19th Century General who served in India and the Afghan War, and died of dysentery in India in 1857, a few days after the end of the siege of Lucknow, where he had been trapped with his troops for over a month.
The other plinth commemorates Sir Charles Napier another 19th century British imperial general and former Commander-in-Chief in India, and a descendant of King Charles II.
In 2000, Ken Livingstone caused controversy by suggesting that the statues of Sir Henry Havelock and General Charles Napier could be replaced by "more relevant" figures, stating "I imagine that not one person in 10,000 going through Trafalgar Square knows any details about the lives of those two generals". For now, however, they remain as a testament to the imperial age and the longevity of statues.
For a more thorough look at the Square, try Londondrum or Wikipedia.