I picked up Arthur the Always King because it is illustrated by Chris Riddell. I enjoy Riddell's style of artwork and this book is profusely illustrated. There is artwork on almost every one of the 237 pages in this book. Some are black and white ink drawings, but there are also abundant full page colorful art and some double page spreads. Riddell must have worked on this project for years. I tried to find a link for this artwork on-line, but could not. Riddell created the delightful Ottoline series and Goth Girl series, which are fun children's books full of great drawings, so check those out if you like Riddell's work.
Unfortunately, the text of Arthur the Always King is not as impressive as the artwork. Crossley-Holland has dutifully narrated the most common tales of the Arthurian legend: the sword in the stone, Gawain and the Green Knight, the quest for the Holy Grail, the betrayal of Arthur by the adulterous Guinevere and Lancelot, the treachery of Mordred - but he fails to make the personalities come alive. I always loved T.H. White's The Once and Future King depiction of King Arthur; White shows Arthur's struggles to make Camelot a paragon of justice, where might does not make right, only to show Arthur's dream failing due to human frailties. The King Arthur in these pages does not come across as sympathetically. I know this is a children's book (though it has a number of bloody violent scenes), so Crossley-Holland is unable to spend time on great depth of character, but it still made the reading experience less than I hoped for. In this book, Merlin explicitly tells Arthur not to marry Guinevere because she is faithless and will fall in love with Lancelot, and Merlin also warns Arthur that Mordred will try to usurp him, but Arthur ignores Merlin, which makes him seem feckless.
Although Crossley-Holland tells us about how Nimue imprisons Merlin, there is no explanation of why she did so. Indeed, after Merlin departs the narrative, so does Nimue and we never learn of her fate.
After pulling the sword from the stone and being crowned King, Arthur announces that the knights of his round table must do chivalrous deeds. Merlin has told him that there will be seven great trials: The trial of friendship and bravery, the trial of love, the trial of honor, the trial of magic, the trial of the Holy Grail quest, the trial of love and loyalty and the trial of the blood knot. The story of Gawain and the Green Knight is the trial of honor. The betrayal of Arthur by Lancelot is the trial of love and loyalty, and because it fails, the round table cracks and Camelot falls.
This book tells that the Roman Emperor Lucius has declared that he will destroy King Arthur. But Arthur gathers an army of a hundred and fifty thousand men and marches across Europe to confront the much larger armies of Lucius. Of course, Arthur is victorious, and he plans to conquer Rome itself. But he turns back because word comes that Guinevere has not been able to rule England properly in his absence. This is the first time in the Arthurian lore that I have read about him leading armies against Rome. But I never read Mallory's Le Morte d'Arthur, so perhaps that part of the story was passed over by T. H. White.
There are many books about King Arthur. The only reason to chose to read Arthur the Always King is if you wish to enjoy the many illustrations.