Read your book thoroughly and note the principal themes and any scenes, sentences or characters that particularly inspire you. Keep a note of the page number where these are so you can refer to them easily again if necessary.
Decide on the main theme for your poem. Do you want to write about the whole story, a particular character or event that features in the book? Or did the story provoke certain feelings or memories within you that you might wish to write about? Remember, you are writing a short poem so avoid trying to write poems with complex plots.
Think about the theme and write in your notebook any keywords or ideas that pop into your head. Refer back to the notes you took while reading the book. Look for recurring themes or words and underline these with colored pens. These are a good base to work your poem around, as these are the aspects of the story that have most affected you. List all the words you have underlined on a new page.
Decide what form you want your poem to take: ballad, sonnet, free verse? There are many forms of poetry. If you are unfamiliar with these, check out other poems in books or on the Internet until you find a form you can relate to and which you feel comfortable writing in. If you are writing in a form other than free verse, read as many poems as you can in that style until the rhythm of them becomes familiar.
Write your poem. This may seem daunting, but focus on what you want to say and keep repeating any rhythms you wish to use in your head. Refer back to the list of keywords and phrases you made. Think about how you might express these in poetic form, what lines come into your head. Keep going until you have a basic draft. At this stage do not try to make it perfect.
Take your basic draft and read it. Underline the parts you are unhappy with. Check back with your original themes and keywords. Does your poem reflect those? If not, but you are happy with it anyway, that is fine. If you feel you have not reflected the ideas you wished to express, refer back to the notes you took when reading the book and reread key passages for further inspiration.
Keep writing and editing your poem until you are happy with it. If you get stuck and find it too difficult to write, take a break and come back to the poem when you are refreshed. Reread your list of key themes and get back to work until you are happy with it.