There is nothing wrong with the Thom book and is virtually the only book to cover the IMC course, except the IMC Confuser, which you should also get a copy of. The only thing in Thom not directly relevant is the section on SIDs, which you do not need at IMC level. If you want something else to help you get your head around ADFs and which way the needle should go then get Martin Cass' book "VOR, ADF and RMI". Read Thom's book in small chunks as you go through the course. It will compliment the flight training and as you get stuck in you will find it facinating.
comparison to getting the basic PPL, how difficult is the IMC
Its only hard in the sense that you learn much more precise flying. Altitude + or - 100 feet, heading + or - 10 degrees; controlling rate of descent to 100 feet a minute. A one hour lesson is a good workout.