So far, none of the recent e-books have been "integral" or indispensable. The first few Typhon Pact
books and The Struggle Within
were all pretty much intended as self-contained, episodic adventures that simply happened to be about a common astropolitical thread -- kind of like, say, how "Journey's End," "The Maquis," "Pre-emptive Strike," "Tribunal," and "Caretaker" were connected by the DMZ/Maquis thread but could all stand on their own as separate installments of separate series. The Vanguard
e-book is an epilogue set after all the print books, so you certainly don't need to read it to follow them. And The Stuff of Dreams
is about as standalone as any 24th-century book gets these days, or so I gather (I haven't read it yet).
As I've said many times, continuity among Trek novels is a bonus added onto the stories, not a basic requirement for comprehending the stories. Each book (or duology or trilogy) is a complete work that can be understood in itself, and reading other books and seeing the continuity ties will just give you a little extra insight into the big picture.