It's easy to lose a book. Something doesn't go quite right, the book is set aside, suddenly, two years have passed, and by now, you're a lot more interested in other books (like the one you're currently writing or the one that's coming up). Years pass. The characters turn into strangers. The book's gone.
I have a ton of books in the drawer. There's the financial trilogy which will likely never happen the way I imagined things in 2008/9. I think I have a novel and a novella there, and both need HUGE amounts of work. There's the menage and its prequel, there's the historical WWII novel (still around, still kicking, and still unfinished) and some other projects that are "cool ideas" at this stage and nothing more. I tend to be drawn towards the new over the old. Research books for the "new" are piling up all around me. You'd have to see it to believe it.
But above all, I'm putting work in to finish with the past and move into the future without guilt constantly dragging at my feet. I think I would be able to move faster that way. I might even tackle ROI again, the first book I did after "Special Forces". While "Collateral" saved me from burnout and from hating writing, ROI was what Germans call a "Befreiungsschlag" - a (violent) move to free oneself (usually from a superior enemy force). For that alone it deserves to be finished up properly. As it's not a romance in the conventional sense, I figure I might have to go down the self-publishing route, but that's OK. I'm done with conventional publishing, anyway.