The Speckled Band references are easy enough to spot, but the story also borrows a lot from Silver Blaze: both features people getting drugged by dinner, an animal gone wild (partly) responsible for a human death and the infamous "negative" clue ("the curious incident of the dog at night-time", or in this case, "the curious incident in the log-book in night-time"

One of the jurors literally says "Silver Blaze"

The love-letter is from one Mary to one James. Mary Marston is Watson's wife (first appearing in The Sign of Four). While Watson is best known as John H. Watson, there's a scene in one of the stories where Mary calls Watson "James", which has been a source of speculation by Holmesians for a long time (one famous explanation is that H. stands for Hamish, which a Scottish variant of James). In DGS4, John Garrideb also notes that his name is John, and not James

The message has four names of people from various nationalities (The Sign of Four