What is our view on the supposed "Lost Books of the Bible," such as the Book of Philip, Book of Mary, etc.? 

IPHC, like almost all evangelical and Pentecostal Christians, only recognizes the sixty-six books of the Bible.  This books have the authority of church history behind them as reflecting the direct inspiration of the Holy Spirit and truly affirming what Jesus and the first century apostles taught.
 
We have always rejected books like those named by you as untrustworthy and fail to meet the historical and theological tests that would make them authentic.  You can read more about this in the chapter on the Bible in my book, Light for the Journey, which can be ordered from LifeSprings Resources. This book was written as an undated elective for Sunday School and small groups. 

Doug Beacham
Executive Director
World Missions Ministries
International Pentecostal Holiness Church