Marlo's output can be quite daunting due to his dry, humourless writing style. He can also be frustrating because his ambition clearly appears to have been to publish (and get credit for) EVERY possible method, rather than publishing the BEST possible method. This tends to lead some people to suspect that Marlo didn't know the difference between a good effect and a bad effect because he only cared about the method... (This obsession also resulted in Marlo's reputation for theft...)
It also means that you can invest energy reading something only to discover that it is totally worthless and you've wasted minutes of your life that could have been spent more fruitfully walking the dogs, washing your hair, colouring in the O's in the dictionary, writing posts for the Genii board, etc.
The upside is that Marlo DID publish some outstanding material. The hard part is finding those items amongst the ocean of dross. Actually this is true of a number of great card creators of the last 30 years who have published hundreds of tricks - a fact which seems to have established their reputation - and through which you must sort to locate the handful of excellent items. Perhaps it's a conspiracy to conceal the good stuff in plain sight? (If only.)
My advice is to get 'Marlo In Spades'. This is generally considered (by the people I respect, at least) to be his most rewarding collection. (The Cardician is ok, though the Spades book is better.) I'd also get his book on false deals, Seconds, Centers, Bottoms, which is acknowledged to be a classic. If the material floats your boat then invest in the Revolutionary Card Technique pamphlets - you can get them from Magic Inc, Denny's and H&R in Texas.
Good luck. And don't lose heart - you can always vary your study (and preserve your mental health) by putting Marlo down occasionally and reading books by authors who tended to only publish material they'd actually use - people like Mike Close, Roberto Giobbi, Darwin Ortiz, Jack Carpenter, and Dai Vernon, etc.