Are there any boundaries between the two generas?
Obviously Science fiction deals with major technological advancements that do not exist in the real world, and Fantasy deals with "in my opinion" more spiritual, magical, and supernatural aspects.
Another question: What are some book recommendations where the two generas are combined?
Scifi and sword & spell stuff are two of many the subsets of Fantasy.
There aren't any objective boundaries, and if you try to set some by making a list of themes that belong exclusively to one or another some author will come along tomorrow and force you to redefine it.
They're useful terms colloquially but not really how they ought to be classed in your head.
One real division might be between "hard SF" (things are meant to be reasonable and theoretically possible under current scientific understanding) vs "SF/Fantasy" (things can be explained as being powered by magic or Beryllium crystals)
The best example I can think of for crossovers would be Dragonriders of Pern. Dragons, castles, swords, flamethrowers, spaceships, bio engineered dragons, lost technology and all that.
There's a lot of interplay between the two genres I'm sure. In terms of literature and not movies or video games I can only think of a short story a friend wrote in college about all the magic on a world being technology based.
There's gotta be tons though seeing all the Final Fantasies, Star Wars, and Magical Girl Lyrical Nanohas there are out there.