Those prequel era books are indeed all still canon, and at least those set between the movies and right before TPM were even Lucas-approved, and some were even demanded to Lucasbooks by G. Lucas (like the one that explains Palpatine's schemes with the TF that leads straight into TPM and that was published shortly before AOTC, IRRC. Lucas wanted it, and he explained the schemes to the writer. It was also one of the better books).