CHT and EBF books are controlled by parameters as found in the personal/prodeo.eng configuration file.
[EBF File = books\dc.ebf] * database location 111 million positions
[EBF Depth = 80] * consult EBF database till 40 moves
[EBF Priority = EBF] * EBF or CHT (search which database first)
[CHT File = books\ccrlcegt.cht] * database location 40 million positions
[CHT Depth = 60] * consult CHT database till 30 moves
[CHT Minimum = 7] * engine classification (0-12) 7 = ELO >= 2900
The [EBF Priority = ] parameter decides which book should be used as first one. For the moment the EBF book performs better and thus the EBF setting is used. When a move is found in the EBF book it is played, if nothing is found it will consult the CHT book. Using the setting CHT will reverse the search order.
The [CHT Minimum = ] controls the minimum (CCRL) elo rating. Each position that is stored in the database is classified by the elo rating of the engine that played the move by a value variying from 0-12. For an engine like ProDeo that is rated around 2700 the minimum elo is set to 2900 (represented by 7) as a safe rating that moves from other programs can be used as book move. The following conversion table is in order to set this parameter to a suitable value that suits your program.