When I first reached for the book I didn't expect it to be a controversial read. Six hours later, I was agape, in denial and most certainly scandalized. Without giving away the plot, suffice to say that Dame Agatha had written her ultimate masterpiece when she decided to write "...Roger Ackroyd".
For first-time readers: Don't be fooled by the length of the novel. The clues are there, sprinkled neatly and merrily along with the darned red herrings. You have to read it slowly, as it was with most of her novels.
The story: a doctor was called by Roger Ackroyd to discuss an important matter, but before he could divulge it further he was interuppted by the evening post. The matter was left there, and the doctor went home, seeing his host a bit disturbed. When he got home another call came in and announced that Mr Ackroyd was dead. Thus Poirot came into the scene and began nosing around. The solution was truly one of the most surprising in literature history.
The novel became the measuring rod for future mystery and detective novels, although its controversy is undeniable. My suggestion is that you ignore the controversy for the moment and concentrate on the story.