I admit it; I am a Seleukid fanboy, and have been for over 40 years. It started with my first Ancients army, which was going to be Ptolemaic Egyptian until I found out that the Seleukids got all the best toys. Over the years I tried a number of times to make my fandom look slightly more respectable by looking for books on the subject to disguise my love for pikes, elephants and cataphracts as a scholarly interest in the Hellenistic period. I didn't have much luck. There was the Bar Kochva book on the Seleukid army (I have a copy of course) and a couple of chestnuts from about 100 years ago, but that was it, other than a few nuggets in WRG publications.
That situation in now changing quite a bit with a number of recent books. Most are specialty publications with price tags well into three digits, but for those of us who are not libraries, there is now a series from the Hellenistic fanboy's best friend, John Grainger. The series now includes THE RISE OF THE SELEUKID EMPIRE, 323BC to 223BC (Pen and Sword, 2014) and THE SELEUKID EMPIRE ODF ANTICHOS III, 223BC – 187BC (Pen and Sword, 2015). These two books, as well as a yet to be published third, will cover the entire dynasty from its rise after Alexander's death to its final demise at the hands of Pompey Magnus.
All of us know the basic history of the Seleukids, but the wealth of detail Grainger is able to wring from the limited sources is astonishing. For example, I had always believed that Antiochos III's great Eastern Anabasis was, apart from a few brief passages in Polybius, pretty much an unknowable mystery. On the contrary; Grainger, by making skilled deductions from multiple sources, is able to piece together a highly credible history of the expedition which fills a gaping hole in our knowledge of the career of Antiochos Megas, the greatest king of the Dynasty after Seleukos himself.
Similarly, Grainger is able to tell us (in the first book) a good deal about the careers os some of the Seleukid kings nobody ever plans a miniatures army around, like Seleukos II and Antiokos II. All in all these books will tell you a great deal about that age-old question; if the Seleukids were good enough on the battlefield to justify their popularity on the tabletop, why didn't they last longer, and why did they have so much trouble controlling their territory? A good deal of time is spent on the causes of dynastic instability in the Seleukid Empire and the measures taken by Seleukid Kings to make the realm more cohesive.
I cannot reccomend these books too strongly to anyone who loves the period.