Is it normal to find unknown work during peer-review process?
It's unfortunate when a referee points out crucial papers you missed in your literature search, and hopefully it won't happen too often.  (The more you talk with other researchers, the more chances you have to learn about these papers earlier in the process.)  However, it's certainly not abnormal.  Nobody can be an expert in everything, some references are really not easy to find, and sometimes you just miss something for unclear reasons.  If you publish enough papers, it's pretty much guaranteed that a referee will occasionally point out something you wish you had known about earlier.  This is one of the advantages of peer review.