Without giving away too much of the plot, "The 13th Tablet" is based around the discovery of an unknown tablet of mysterious significance. The book's heroine, Mina Osman, discovers the tablet through a happy accident (or a serendipitous event, if you prefer) which turns out to be not so happy after all. The discovery leads to an international chase for both the tablet and the truth, with a shocking climax.
The book is a real page-turner. The story is exciting and fast-paced, and there is some very interesting historical background underlying the plot. It's a very enjoyable read, and both I and my mother found it fascinating and finished it quickly. We'll be looking forward to the next book in the promised trilogy.
On the negative side, the book is possibly too fast-paced and doesn't take quite enough time to fully establish the characters. A bit more description would add some more colour to the narrative, and I would have liked more information on the historical background. It's there, and clearly a lot of research was done into the background subjects, but it takes second place to the action and I felt that it was certainly interesting enough to deserve more space. More information is available on the author's website, though.
These criticisms do not detract from the overall enjoyability of the book. It has a great plot and lots of action, an interesting central character, and lots of drama. It's certainly worth a read if you like action stories with a sound historical background.