I have a copy and use it a lot even though it was first published in 1891 written by H. Denis Taylor .The basic principle of astronomical objectives ( the front lense or mirror) hasent changed much in 119 years. so even as this is an older book the telescope today is very similar to the telescopes then.
The main changes are the types of glass used today are of much better quality optically ,thats about all! and computer designed optic paths,But a human in final figuring is still needed.
So this hand book is well written and easy to understand if you have been around telescopes for a while and dont be suprised if a 6th edition emerges soon ,the last was 1983 published by Adam Hilger LTD., Bristol .