I am getting ready to install some bamboo (19/32 x 3 3/4 x 37 3/4) over 3/4 plywood that is on floor joists spaced at 24". I plan on going perpendicular to the joists. Will this be ok? I will also put down some 15# felt. Do I need to put down another layer of plywood? I really don't want to go too much higher. Thanks for all your help. This site is awsome!

That 24" OC for 3/4" plywood is the maximum span for 3/4" ply. You most likely are now getting some deflection between the joists. That may cause noises and fasteners becoming loose over time. Bamboo doesn't have the rigidity that solid hardwood flooring does so it will not add much to the strength of your subfloors, like a solid hardwood can. IMO, adding an additional layer of 3/8" minimum CDX plywood over your existing would be the wise thing to do.