TOEFL

Some of the best TOEFL preparation books are:

• The Official Guide to the TOEFL iBT Test, 6th Edition: Made by ETS, it has real practice questions, sample essays, and 4 full practice tests.
• Barron's TOEFL iBT: Gives 8 full practice tests, listening audios online, and reviews of all TOEFL sections.
• Kaplan's TOEFL iBT Prep Plus: Has 4 full practice tests and 450+ sample questions with answers.
• Cambridge Preparation for the TOEFL iBT Test: Has many exercises, tips, and 7 mock tests for practice.

The minimum TOEFL score is not same for every university or program. It change depending on where you apply. In general, many top US universities ask for a TOEFL score between 80 to 110 for undergraduate and graduate programs.

For example:

• MIT (Massachusetts Institute of Technology): 100+ for Bachelor's, 90+ for Master's
• Harvard University: 80+
• Stanford University: 101+
• Caltech (California Institute of Technology): 110+

The difficulty of TOEFL compared to IELTS depends on individual strengths and preferences. TOEFL is generally considered more challenging in terms of integrated skills, as it combines reading and listening with writing tasks. However, IELTS may be more difficult for scoring high in productive skills due to its analytical scoring system. 

While the standard TOEFL iBT is commonly accepted, not all German universities explicitly accept TOEFL Essentials. It is recommended to check individual university requirements on their official websites to confirm acceptance of this format.