S. Granger is a classic but you can get much more to read off the Net. So yes, I suggest a few searches.
> Is this topic suitable for research? Me and my co-researcher
> are going to collect learner corpora of Graduation Level.
There are conference chains devoted to this topic, cf. e.g. http://calico.org/
When designing your corpus, do devote some time to its format (and think of going beyond ICLE).
Here's one relevant example of a learner corpus with solid, well-thought design: the FALKO corpus of learner German -- try
(a presentation, but you can find a paper on FALKO on the Net as well)
I have designed a prototype in TEI, reported on in the proceedings of BUCC-2010 ( http://www.fb06.uni-mainz.de/lk/bucc2010/ ), but that should only be treated as food for further thought. FALKO, on the other hand, is a working, searchable resource.