HelpStudio 2011: I want to create a PDF-booklet with klickable links to other topics in the same booklet. Which formatting-string do I have to put in the field "Replacement String" (window "Link Conversion" in the booklet properties)?

You can enable clickable links for links to Topics in the project by un-checking the "Replace links to Topics that are included in this Booklet" option on the Link Replacement page in the Booklet editor.