Hello eseq;
The "bible" for S7 programming is by Hans Berger, Automating with Step 7 in STL and SCL.The ISBN for the 2005 edition is ISBN 3-89578-243-2 (many thanks to KRK for this info).
There is also, by the same author, Automaing in Step 7 in LAD and FDB.
Both are published by Wiley; have a look on Amazon, for example.
Hope this helps,
Daniel Chartier