Theory:

I spotted it in a junk shop in Bridport, a roll-top desk. The man said it was early nineteenth century, and oak. I had wanted one, but they were far too expensive. This one was in a bad condition, the roll-top in several pieces, one leg clumsily mended, scorch marks all down one side. It was going for very little money. I thought I could restore it. It would be a risk, a challenge, but I had to have it. I paid the man and brought it back to my workroom at the back of the garage. I began work on it on Christmas Eve.
 
I removed the roll-top completely and pulled out the drawers. The veneer had lifted almost everywhere — it looked like water damage to me. Both fire and water had clearly taken their toll on this desk. The last drawer was stuck fast. I tried all I could to ease it out gently. In the end I used brute force. I struck it sharply with the side of my fist and the drawer flew open to reveal a shallow space underneath, a secret drawer. There was something in there. I reached in and took out a small black tin box. Sello-taped to the top of it was a piece of lined notepaper,  and written on it in shaky handwriting: “Jim’s last letter, received January 25, 1915. To be buried with me when the time comes.” I knew as I did it that it was wrong of me to open the box, but curiosity got the better of my scruples. It usually does.
 
Inside the box there was an envelope. The address read: “Mrs Jim Macpherson, 12 Copper Beeches, Bridport, Dorset.” I took out the letter and unfolded it. It was written in pencil and dated at the top — “December 26, 1914”. 
Explanation:
  
The author has visited a second-hand goods shop in a place called Bridport. He saw a roll-top desk, which he had been needed for a long time. The ones he saw very costly, but this one came very cheap - because it was damaged. The roll-top was broken into pieces, one leg was awkwardly broken, and it had fire burnt marks on one side. Repairing it was a huge challenge, but he was willing to take it up because he wanted the desk for long, and the seller said it was oak and it belonged to the nineteenth century. He paid the money and brought it to his house and started repairing it. It was the day before Christmas when this happened.
 
 He removed all the parts of the desk completely and pulled out the drawers. The thin polish on the wood had worn out as if water had damaged it. The damages to the desk were caused both by fire and water it seemed. The last drawer was shut hard. He tried gently at first, then used hard force, hit with his fist and the door flung open. There was a shallow space, a secret drawer underneath. He reached in and took out a small black tin, which had a sello-taped letter, written in a notebook paper. The handwriting seemed shaky. It said, "Jim’s last letter, received January 25, 1915. To be buried with me when the time comes."
  
The author knew he should not open it, as it belonged to someone else. His morals stopped him from opening it, he hesitated for a minute, before curiosity won him. In the battle between values and morals versus curiosity, curiosity usually wins.
  
Inside the box, there was an envelope, which read "Mrs Jim Macpherson, 12 Copper Beeches, Bridport, Dorset." He took out the letter and saw that there was a date written in pencil on the top - "December 26, 1914", This means that the letter was originally written on December 26, 1914, and had been revisited on Jan 25, 1915. Someone had made the note about it being Jim's last letter, which had to be buried with them.
 
 
Meanings of difficult words:
  
Words
Meanings
junk shopa shop selling second-hand goods or inexpensive antiques
roll-top deska writing desk with a flexible cover sliding in curved grooves
clumsilyawkwardly
mendedrepaired
scorchburn the surface with flames or heat
going for(here) selling for
restore(here) repair
taken their tolldamaged
stuck fastshut tight
ease it outto move out carefully and smoothly
brute forceapplication of planned force
scruplesfeelings that make you hesitate to do immoral things
veneera  thin decorative covering of fine wood applied to a coarser wood
 
Reference:
National Council of Educational Research and Training (2008). Honeydew. The best Christmas present in the world - Michael Morpurgo (pp. 9-16). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.