I have created a custom search engine: Search Spiritual Books - India

Sites added are

Sites Refinement labels ramakrishna ramakrishna ramakrishna ramakrishna ramakrishna gitapress samata motilal parimal amazon

Clicking Refinement Labels will filter search result from sites to which that particular label is applied

e.g. searching for 'bhagavad Gita' and clicking on refinment amazon will search only

Alternatively if you wish to add a refinement filter in search query itself then type

bhagavad gita more:amazon
If you know any particular site that is very useful, please let me know and I will add it.

Please note that I can have only 16 refinement labels.