"楔形文字的" can be translated as "cuneiform" (when referring to something related to or characteristic of cuneiform writing) or "of cuneiform script" (for a more descriptive phrase).
For example:
楔形文字的碑文 → cuneiform inscription
楔形文字的符号 → cuneiform signs/symbols
一本关于楔形文字的书 → a book on cuneiform script/writing