Are trust sederunt books a good source for family history?

Bear in mind that the proportion of people who left trusts is small, and the proportion of trust sederunt books which have survived from the 19th century is even smaller. If you are fortunate to find a sederunt book surviving for an ancestor, then it will be a valuable source of information, but principally about his property and lifestyle. It is unlikely to give you information about the family which is not available from other sources.