Like EndNote 7, the current version of EndNote (X3 )also does not permit formatting (i.e., capitalization, lower-case) for fields within the individualized style templates - which is needed to address your problem. Since this option is unavailable, the next to consider is manually capitalizing the book titles in the corresponding entries in your library. This will generate the capitalization of book titles but will create other problems when/if you later need to generate a list where sentence style is preferred.
Another option might be to have the book entries in your library maintain the sentence style capitalization and insert them into your document as usual along with generating the reference list. Then convert the document to text (to remove the EndNote field codes (save a backup copy of your original document before doing so as once the codes are removed you won’t be able to put them back in). After the document is converted to text, you could manually go through the list and use MS Word’s capitization option to change the sentence style book titles to uppercase. (Assuming you are using MS Word and it may depend upon what version you’re using.)
Not a neat fix but until EndNote allows formatting capability for fields within the style templates, you may need to resort to creative self-solutions. (You can also toss the idea on the product suggestions part of the EndNote forum for the developers to consider.)