Important historical type with the name of Sejanus removed in damnatio memoriae.
I have been browsing through two Tiberius biographies, Tiberius by Robin Seager and Tiberius the Politician by Barbara Levick, to see how realistically Sejanus has been portrayed in the novel Tiberius: The Memoirs of the Emperor by Allan Massie, our current read.
Indeed, it seems the connection with Tiberius is almost as early as imagined in the novel. Both biographers indicate that they accept Tacitus' depiction of Sejanus. Both however cast doubt on the assumption that Sejanus had Drusus poisoned, picking apart the story of the letter which Apicata, the divorced wife of Sejanus, sent to Tiberius after the former's death. But, as Seager writes, the issue is really academic: Drusus had to die and die soon. Thus, if he hadn't died of natural causes, he would have been eliminated sooner rather than later.
I do agree with Robert, who recently commented on the Levick book: The author assumes prior knowledge of the subject by the reader. The death of Drusus is discussed but not actually reported on. Levick jumps back and forth, whereas Seager is much more accessible and covers the rise and fall of Sejanus in its own chapter in a cohesive narrative.
Readers who enjoy the Massie book might also like Sejanus by David Wishart, one of the "political" novels in his Marcus Valerius Covinus Mysteries series.
Then, there are Ben Jonson's drama Sejanus and Sejanus, and Other Roman Tales (1839) by Edward Maturin at the Internet Archive.
Sejanus at Smith's Dictionary
ancient historians on tiberius (a.d. 14-37) (at this site)
Sejanus at Wikipedia, nicely illustrated (as usual, handle with care)