It would appear that the value list is entering different data than "Book" when you select from that value list. Is this a custom values value list? If so, check for an extra space before or after the value. Is this a "use values from field" value list? If so, perhaps the text "book" is from the second field and your drop down list is actually entering an ID number.
If that's not it, try committing records after selecting from the value list by clicking the layout background. If that's all that's needed, a script can do the same from a script trigger set on the field.
Thank you so very much Phil! It was a custom value list and when I went back and had a look at it, I had put a space after the word "Book". I spent ages yesterday trying to figure out what was wrong, but everything works perfectly now. I really appreciate the help.