Thank you for clearing that up. So it would be correct to say that
the main difference between the two ts and the two ds is the place
of articulation and the resulting presence or absence of
aspiration. Then De Silva is certainly confusing on that point, not
least because the difference between 'th' in 'thumb' and 'they' is
that one is voiced and the other is not, which is a separate issue.

Would it be right to say that double consonants are pronounced
double (eg as in Italian)? This point is not mentioned in either
book.

Thanks!