Shuddha-advaita is a school of Vaishnavism. The sect is known as Rudra or Rudza. It was founded by Vallabha (first half 16th century). It differs from the Advaita school followed today primarily known through Adi Sankara's teachings.
Hinduism Today magazine (July 2008 issue and available here - http://www.hinduismtoday.com/) states:
This valuable summary is drawn from The Sri-Krsna Temple at Udupi, by B. N. Hebbar, who notes that “all five schools [of Vaishnavism] are theistic and realistic reactions to the absolutistic idealism of Shankara’s Advaita Vedanta…. The first two are South Indian and follow the aishvarya bhakti-marga (Master-servant relationship between the Lord and His
devotee), while the latter three [including the Rudra sect] are North Indian and adhere to the madhurya bhakti-marga (Lover-beloved relationship between the Lord and His devotee). Also, while the Lakshmi-Narayana concept predominates South Indian Vaishnavism, the Radha-Krishna element pervades the three North Indian Vaishnava sects.
Basically, being a Vaishnavite, Vallabha sees the universe as real and rejects the concept of Maya (or Vivarta Vada, apparent manifestation) of Sankara. Although Vallabha admits of spiritual union with Brahman, being a Vaishnava sect, it sees the love of Krishna and the mystical union with Krishna as the highest spiritual goal.
There are two articles in Hinduism Today. On their website do a search for 'shuddhadvaita'. There is also a 1 page summary of Vallabha and the Rudra sect in The Spiritual Heritage of India by Swami Prabhavananda. You might also try a Wikipedia search on 'Vallabha' although the info here is historical and not philosophical.
There is another very good source I remembered. Swami Vireswarananda's translation of the Brahma-Sutras Sri Bhasya has an excellent and long introduction (80 pages) where he goes into the different commentators (Sankara, Ramanuja, Madhva, Nimbarka, Vallabha, Bhaskara) differences in their specific interpretations of Brahman, the Soul, and release from this world and how each commentator uses specific Sutras to support their individual positions. I do not know of a softcopy on the internet.