Возьми какого нибудь автора и его произведение. Вот примерный текст. Можно еще написать несколько предложений о произведение) Удачи)

Hello, dear writer! I thank you for your book. This book has become my new discovery. It is very interesting and informative. Another thing I really want to read your next book. I think it will be as interesting as the previous book. Thank you very much for what you write such wonderful books. I wait for a new book. Thank you again to read my letter. I wish you plenty of new discoveries and inspiration.

Мне думается, что подобное письмо следует связать с каким-нить событием,  например с Рождеством. Поэтому, если вы сочтете мое письмо недостаточно информативным, вы можете добавить поздравления, каких-то пожеланий и т.д. Надеюсь, вас не взяли во временные рамки и поэтому напишем письмо ныне покойному Артуру Конану Дойлю.

London

April 11

Dear Arthur,

Hope all is well with you.

It was so good to read your book a few weeks ago. I really got indescribable pleasure from your book! I don't know from what begin!

Sherlock Holmes' character is not as common as any investigator in life. He reveals an intelligence and cleverness personality. I feel a real respect for him. It's amazing to look on his deductive thinking and, in the end of the story, understand how it easy at all! But i think you pay too little attention to Doctor Watson. And, in my opinion, you should write a few stories with his direct participation, perhaps, with him in title role.

By the way, i am coming to London next week, and it would be a great honor for me to spend a few days with you! I have dreamed for meet with you since i read my first book about Sherlock Holmes! I am looking forward for your answer to me!

I can't wait to hear from you, really!

Best wishes

Your name (замените это на ваше имя)

Спасибо за внимание, я счел нужным не перегружать письмо потоком эмоций, а лишь вкраце выразить восторг автора письма, что ему понравилось в главном персонаже, и его пожелание на будущее, и разумеется, пожелания автору.