I searched for an open specification of the peercoin protocol (more precise than the white paper) and I couldn't find one. Is it possible to find something similar to the bitcoin protocol specification available on bitcoin wiki but for peercoin ?
Since most crypto-currencies are a fork of Bitcoin, you can get nitty gritty technical details from the Bitcoin wiki, specifically from the technical category.
Say you want to know the specifications for communication, the Protocol Specification article will help you out with how packets should be formatted down to the bit. It will have all sorts of details on things like how to calculate difficulty or what a block chain is.
Peercoin share a lot of concepts with Bitcoin and thus most Bitcoin wiki articles will apply. Peercoin concepts such as its Proof-of-Stake may not be found in the Bitcoin wiki so refer to the whitepaper for differences.
This is old and the wiki is being redone but I used this when dev'ing peercoin related projects. http://wiki.peercointalk.org/index.php?title=Main_Page
You could treat the code as a precise specification of sorts. Eg: https://github.com/peercoin/peercoin/blob/master/src/kernel.cpp#L426
Its pretty terrible that they don't have a well-written spec for peercoin. It never really caught on tho, so not a whole reason to delve that deeply into it. There are better PoS designs these days if you're looking for ideas.