The core team sent word this morning of a new release numbered 1.5.5a. This comes only 2 months since the release of 1.5.5. 

Looking at the fixes, none were crucial as far as security goes but there are a couple of things that were obvious problems for users and a few just plain boo boos. 

That list is located at the bottom of the page here: https://www.zen-cart.com/showthread.php?219730-Zen-Cart-v1-5-5a-released!  It does reference the entire Known bugs and fixes in v1.5.5 thread as well.

I still recommend holding off updating in case there are more issues that crop up in the coming months.