I love St. Alphonsus Maria de Liguori! He's a saint quoting saints! I haven't read that book you mentioned Jackie, but doesn't it contain at least some of his writings too? Or entirely his in various extracts?
We have so many books on saints books and I love each one. If you opened a few and read a few first paragraphs of several books perhaps you would find something that touches your situation to hold onto now!
I go visit
Saints' Quotes regularly and it helps me so much and answers my spiritual questions of the day! I highly recommend it for getting your spiritual life on track! God may bring just the right quote you need to hear at the time to you!
I agree the Interior Castle is likely a bit too deep for us here, I think, at this stage in our spiritual lives. But it helps stretch us to make a beginning towards learning more! I think when we start a new book study we will go for a very simple easy to read choice next. We already are doing Fr. Furniss and he is very simple and beautiful, thankfully.
Perhaps you should try one of his? There's the Imitation of Christ too, that many people have been helped by, it's rather like the Sinner's Guide. Also you might find Introduction to the Devout Life helpful.
I am always going back to St. Alphonsus Maria de Liguori's 'The Way of Salvation and Perfection'. And I will go back and read Rev. F.X. Schouppe, S.J.'s 'Purgatory' too.
Fr. Paul O'Sullivan, O.P. is also very easy to read and intensely helpful.