As a Spanish historian, I frankly don't see this in the historiography of the Spanish Civil War. The death of Jose Calvo Sotelo is almost always discussed as one of the immediate causes of the Spanish Civil War. Frankly, any book that doesn't discuss it is a poor book.

I suppose if a book did leave out his death, it would hint at the politics of the writer. The murder of Jose Calvo Sotelo tends to be more emphasized in books that are sympathetic to the Spanish right, given Calvo Sotelo's prominent role in the monarchist party, "Renovacion Española". If you are a leftist, however, you are more likely to point out that the murder of Calvo Sotelo by the Republican Assault Guards was a retaliatory response to the murder of Lieutenant Jose Castillo, an Assault Guard and ardent Socialist. Thus, an omission of Calvo Sotelo could potentially raise questions about the politics of the book and would suggest that the historian is more sympathetic to the Republic. But I hardly ever encounter this. The more common pattern I find is that Calvo Sotelo is singularly emphasized, or his death is qualified by bringing up Jose Castillo.

If you wouldn't mind sharing with me some of the titles that omitted the death of Calvo Sotelo, I'd appreciate it. It's possible that some of the books you read approached the Spanish Civil War from a longer perspective, addressing more the long-term structural and social causes of the war rather than its immediate causes.