Jim's 1000: Listening Comprehension Practice Test Items for the New TOEIC Test includes 10 practise tests with detailed expression reviews and answer keys. The Jim's 1000 TOEIC practise test book is a must-have material that you should have prior taking to a real Toeic test. The practise tests are almost similar in difficulty level of a real Toeic tests, especially in contexts, vocabularies, question types, question lengths, and answers.
If your current score is 600 and your target score is above 800, you can start to practise this book from now. 1 test per week, then increase to 2 tests per week. After 2 months, you can take the real test confidently. Good luck!