Assalaam o alaikum
This is well known with the name of "Tabaqaat Ibn-a-Saad", written by Muhammad Ibn Saad, in 9th century. Because He was the "Katib" of "Waaqdi" so it is assumed that the context of the book belongs to Waaqdi, but famous by the name of Ibn Saad, Allah knows better.
This is a book of "Ilm-ul-Rijaal", means biographical informations - it is consist of 8 books, first two belongs to "Seerat-a-Muhammadi" and remaining consist of biographical information about famous personalities of Islam up to 9th century.
S. Moinul Haq is not the author of the book but the translator, the book is translated in english by Mr. Haq.
And as the matter of its importance in Islam, then it is a very normal category book consist of all kinds of informations i.e. useful, worthless, wrong, able to consider, blunder, so and so.
regards