Are you looking for information on Yammer? The 1st thing to know is that with Office 365 – it is an ‘evergreen’ solution that is constantly reaping the benefits from all users recommendations/bug fixes/updates. So with the ever-updating Yammer application the best place to find information about Yammer is still the help from the app or at https://products.office.com/en-us/yammer/yammer-overview
Yammer Admin Guide (https://support.office.com/en-us/article/Yammer-admin-guide-a9c206d4-fb18-4250-bec6-e783b926e4f6).
Yammer User Guide – not that I know of – however there is great help once in the app.