I recommended the Paper Engine, as it is a read that will allow you to think about your magic. The lessons in it extend past the effects. I do own and have read all the books you mentioned.
Even though I recommended the Paper Engine, I would have to know why you do magic, what you do with it, and what kind of level/stuff you do now - to really give you a better answer.
From my stand point - By Forces Unseen has some fun toys, and smart design, but I feel most of it is overhandled or is so difficult that you will spend an tremendous amount of time to perfect it and then not be able to use it in working situations all that often, minus a few effects. That being said - it is still a great book, with some very untraditional thinking, if you want to do some knuckle busting, it will keep you busy. I do a few effects out of it, but find most of it not all that "entertaining" in comparison to other stuff - MOVE MONKEYS love this book, and I don't blame them.
The Classics of Larry Jennings - I have a first edition of this book, and the material in it is very solid. The effects range from coins, cards to cups and balls - but much work is done on the table. Jennings was one of the best, and depending on your style, this book can really be a useful lesson in some interesting methods and sleights. I don't use much out of this book, but I still enjoyed going throug it.
Drawing Room Deceptions is a great read - Guy's work is really clever and I use many of his effects. I learnt Reformation from video, so found it easier to read the method from the book...but from those without that advantage, I hear learning it from the book will break your brain! Many of his effects are very usable in walk around settings, but take some practice...Guy makes it look easy, as his hands are the size of a small child. However, all his routines are very well constructed and entertaining.
As for Paper Engine - I use a few effects from this book - Aaron thought this book out well...actually, even in the book he will tell you why something sucks and why it was created to solve and the problems it created. He includes stories, and related "after thoughts" that are bang on...some inspired me. Depending on where you are in your magic, you will get something new out of this on each read. He made this book with "The magic of Paul LePaul" in mind, and you can see the influence in design, but that is it. In my opinion, the Paper Engine takes it up a notch in what it has to offer a modern reader. (Please don't take this as a knock on the LePaul book - as it is a GREAT and CLASSIC book)
There - I hope that helps you make an informed selection.
