Can you pleas help me check these sentences, please? Thank you very much.

1) The book tells about/is about/deals with (are they all correct?) a boy who was killed in a car accident.
2)I liked this year's Christmas holidays because they were very long.
3)I spent Christmas day at home with all mu family. My mother cooked the Christmas lunch and we ate a lot.
4) I enjoyed the salmon my aunt cooked and the stewed (?) lamb of my mother (which my mum prepared).
5) On New Year's Eve I had a party with my friends and I really had a good time.
6) From 3 January to 5 January I spent three days in the mountains with my friends. I enjoyed myself because I sang, played on the snow (with the snow?) and skated on ie with my friends.
8) On 6 January I played bingo with people from my parish.I returned to school on 10 January. My holidays were long but went by very quickly.
9) Can you say "if he had this word written on paper", he would tear it.

1) The book tells about/is about/deals with (are they all correct?) a boy who was killed in a car accident.

Since books don't talk, I'd ditch that first one! The other two phrasings are fine.

2)OK

3)I spent Christmas day at home with all mu family. My mother cooked the Christmas lunch and we ate a lot.
"my" not "mu" -- comma after "lunch"

4) I enjoyed the salmon my aunt cooked and the stewed lamb which my mum prepared.

5) On New Year's Eve I had a party with my friends and I really had a good time.
comma after "friends"

6) From 3 January to 5 January I spent three days in the mountains with my friends. I enjoyed myself because I sang, played on the snow (with the snow?) and skated on ie with my friends.
comma after intro phrase; "played in the snow" and "ice" (not "ie").

8) On 6 January I played bingo with people from my parish.I returned to school on 10 January. My holidays were long but went by very quickly.
comma after intro phrase; space after period after "parish"

9) Can you say "if he had this word written on paper", he would tear it.
I don't understand that. If he had what word? Only one word? Would he tear up the word or the paper? ??

Sure! Here are the corrections and suggestions for your sentences:

1) The book is about a boy who was killed in a car accident. (All three options - tells about, is about, deals with - are correct and convey the same meaning.)
2) I enjoyed this year's Christmas holidays because they were very long.
3) I spent Christmas day at home with my whole family. My mother cooked Christmas lunch, and we ate a lot.
4) I enjoyed the salmon my aunt cooked and the lamb stew my mother prepared.
5) On New Year's Eve, I had a party with my friends, and I really had a good time.
6) From January 3rd to January 5th, I spent three days in the mountains with my friends. I enjoyed myself singing, playing in the snow, and skating with my friends. (Using "with the snow" is also correct.)
8) On January 6th, I played bingo with people from my parish. I returned to school on January 10th. My holidays were long but went by very quickly.
9) Can you say "if he had this word written on paper, he would tear it"? (This sentence is grammatically correct, but the meaning might depend on the context.)

I hope these revisions help! Let me know if you have any further questions.