Does publishing a monograph with Springer in computer science have any academic value?
Is there any academic value in publishing a book that is based (approx. three quarters of it) on existing (peer-reviewed) material?


If well done, this seems like a useful book to me. Bringing together various related papers, adding some expository material for readers not yet intimately aware of the field, providing some perspective through use cases ... why not? Remember that in CS, the point of books is almost never to have original material. Books in CS are meant as a collection / introduction to a topic, not as a venue for new research contributions (that's what we have conferences for).


  Also, as I am considering applying for assistant professor positions (I am currently working on contract basis), I would like to ask how is this book monograph viewed at by hiring committees? Is it considered vanity or a plus?


Springer isn't a typical vanity press outlet, so I can't see a hiring committee outright discarding your publication there. I guess it will be a small plus (or a bigger plus, if the book turns out really good and becomes better-known in the field). However, your actual original research will probably be much more significant.