My favorite way to join cabinets is to not need to join cabinets. I've installed entire kitchens with big cabinets and not a single "joined" cabinet except corners. Bigger cabinets are no fun to carry, but easier to install and often improves the look, depending on the style.
But for the times I do need to join cabinets, I try every trick I can think of to hide them--behind hinges, in drawer banks, pocket screws on outside of carcass. Sometimes with inset doors, the best place to hide a screw is the faceframe edge on the hinge side--easy to plug, and really doesn't show anyway.
For open bookcases, sometimes glue and clamps can work.
I use about 7 trim head screws a year, usually to wrestle some warped piece of trim into place when nothing else is working.