Did it look anything like this? If so, it's a soldier's soldbuch (pay book), which contained the individual's photograph, vital statistics, list of issued gear, and personal history (where he did his training, what units he had been assigned to, promotions, any special qualifications, etc). It was required to be carried on the individual's person at all times.

These instructions (roughly translated) were printed on the back of the cover:

 1.  The Soldbuch serves the soldier as a personal identification in wartime and as an authorization to receive pay from his own or outside pay-stations.  In addition, it is an identification for rail travel, detached service, leave, and for receiving mail.
 2.  The Soldbuch will always be carried by the soldier on his person in a tunic pocket.  Leaving the book in one's baggage or in one's quarters is not permitted.  The careful preservation of the book is in the best interest of the owner.
 3.  The Soldbuch must be kept in an orderly fashion.  The owner must see to it, that all changes in pay due to transfer or promotion are immediately entered by his responsible duty station.
 4.  The Soldbuch is an official document.  Entries are only to be made by a Wehrmacht duty station.  Making unauthorized changes is punishable as falsification of official documents.
 5.  The loss of a Soldbuch is to be reported as soon the loss is discovered to the holders unit or duty station, and the issue of a new Soldbuch will be requested.

I'm indebted to this website for the translation, as well as a general analysis of the soldbuch and its contents.