Bismillah-ir-Rahmaan-ir-Raheem

The book you refer to is an English translation of a highly reputable, classical manual on Shafi'ee Fiqh, 'Umdat-al-Salik wa Uddat-un-Naasik' (The Reliance of Traveller and Tools of the Worshipper) written by Sheikh Ahmad Ibn Naqib Al-Misri (Died - 769A.H / 1368 C.E.)

The English translation by Nuh Ha Mim Keller has been authenticated by Al-Azhar university and other Ulama. (See introduction to the revised edition by Amana Publications).

There is no harm in Shaf'iee laymen studying the book provided they refer to a qualified and reputable Sha'fiee Mufti in personal fiqhi matters.

As for Hanafi laymen, it is not advisable to study the book since it can cause confusion and lead one towards Talfeeq, i.e. picking and chosing between the different Maz'ahib. This is Haraam as it involves probing in matters of Shari'ah without possessing the authority to do so.

This type of home made 'Ijtihaad' eventually leads one into the trap of Shaitaan and one's Nafs. (Allah forbid!)

Allah (SWT) Knows Best.

(Mufti) Abdullah Patel