This book is a period piece. Whilst all three books have flashes of excellence, they become increasingly rare as you progress. Each book may have the same number of good parts, but they get more and more separated by huge sections of turgid philosophising.
You are of course aware of Lewis' strong Christian beliefs at all times, but some parts are just awful. The ending of Perelandra in particular with page after page of paragraphs in the structure 'He who... ..Praise Him'. I had to simply skip. What's more, the nature of some of what he suggests Jane Studdock's views and behaviours should be (as opposed to what they have been) are simply unacceptable in a modern world.
Read only if you have a strong stomach and a lot of patience, the flashes do not justify the whole.