Fundamentals
General valuation / accounting know how
Risk free rates and discount rates for DCF
How to calculate Enterprise value
Operating Cashflow and interest expenses
Operating Leases and IFRS (German)
Special situations
Underrated Special situation: Deeply discounted rights issues
Some thoughts on holding companies
3 part series on European spin offs: part 1, part 2 and part 3
Preference shares (German) part 1 and part 2
Portfolio construction / management
Position sizing and cash quota
Leveraging Investment returns if you are not Warren Buffet
Behavioural Investing: Averaging Up vs.Averaging down
General thoughts
Competitve advantages of individual investors
value and opportunity 
Easy to understand and informative