Well, there is a lot out there on the Seleucid Empire, though some aspects of the empire are better known and researched than others. The Wikipedia article and the links found therein are a good start. The Livius page is also good, especially if the follow the rabbit hole and click on any links that might interest you. Livius is especially useful for providing information regarding Near Eastern documents.

As for books, there are decent chapters in most general histories of the Hellenistic World. Peter Green weaves information of Seleucid kingship throughout his book, but you need to slog through entire chapters in order to find information specific to the Seleucids. I greatly prefer Graham Shipley's book on the Hellenistic world, which has its own chapter on the Seleucid kingdom.

There are more specialized books out there, but you may need access to a university library in order to find some of these books. Susan Sherwin-White and Amelie Kuhrt made a splash amongst ancient historians when they published Samarkhand to Sardis nearly twenty-five years ago. The book was the first book-length attempt to situate the Seleucid Empire in a more Near Eastern context, with particular focus paid to primary sources in languages other than Greek and Latin. While the book is a bit dated now, it is a must-read for any serious historian of the Seleucids.

More recently, Paul Kosmin published The Land of the Elephant Kings. This book is a good attempt to reconstruct the intellectual and royal levels of society within the Seleucid Empire. The research is top-notch and up-to-date, and I highly recommend it.

If you have any particular areas of the Seleucid Empire that you wish to more know about, let me know. The academic scholarship is immense, even if much of it is not available to the general internet.