Try it without WHERE
and use IFNULL
to replace null values with empty string:
SELECT IFNULL(bk.id,'') AS book_id, IFNULL(im.id,'') AS image_id, IFNULL(im.file,'') AS filename, IFNULL(bk.title,'') AS book_title, IFNULL(im.description,'') AS img_desc
FROM book_table AS bk
LEFT JOIN image_book_rel AS bi
ON bk.id = bi.book_id
LEFT JOIN image_table AS im
ON bi.image_id = im.id
ORDER BY bk.id
Result:
BOOK_ID IMAGE_ID FILENAME BOOK_TITLE IMG_DESC
1 Book 1
2 20 book_img2.jpg Book 2 Image of Book 2
3 30 book_img3.jpg Book 3 Image of Book 3
4 Book 4
5 50 book_img5.jpg Book 5
6 Book 6
7 70 book_img7.jpg Book 7 Image of Book 7
See result in SQL Fiddle.