Fortunately, we still have de Rojas' testament and lately Rafael Gomez Diaz, the municipal archivist of Talavera de la Reina, presented to the scholars a document written by Fernando de Rojas that appeared to have the same idiolect as his testament. We therefore know the idiolect of de Rojas and thus will be able to trace his possible books that could have been written under a pseudonym or anonymously. The first volume carries the first 19 of the 49 books de Rojas owned according to his testament. So far, I only detected the idiolect of de Rojas in 5 of these 19 books. The JGAAP is a very helpful program, but using it is not enough to be sure that the results are 100% reliable. For that reason, we need additional proofs, such as studies by other researchers and certain words frequently used by the scholars with the help of CORDE. Taking the mysterious writing activities of Fernando de Rojas and Gonzalo Fernandez de Oviedo into account, the goal of this book is to provide a toolbox from which researchers can draw conclusions about the possible works of these two authors.