The Library chair is a fun, straight forward project that will be handy in any household or shop. I constructed the chair/ ladder using hardwood which does add a little weight but the strength is greatly improved with a hardwood.
All joinery is mortise and tenon which could be pinned if you were concerned about the strength of the joints. I don’t believe the normal M & T needs to be reinforced as long as the joints are snug fitting. The chair sits 16 ½ ” from the floor which is about average for seating and the riser distance is basically 8 ½“ which is also fairly standard for a ladder. The distance between the top of the ladder and the highest step is a little less, but you shouldn’t be standing there anyway.
See the Library chair in action here