Hi Jim,
I'll post what she (the author) says:
When the TH combination connects with certain sounds, te two sounds blend together to form a composite sound. In the following examples, see how the TH moves back and the L moves forwars, to meet in a new middle position. Repeat after me:
th + l -with lemon
th + n - with nachos
th + t - both times
th + d - with delivery
th + s - both sizes
th + z -with zeal
th + ch - both charges
th + j - with juice
n + th - in the
z + th - was that
d + th - hid those
If you speak slowly, maybe you can pronounce THs and the other sounds distinctly, but at normal speed or when speaking fast (you Americans often speak fast, don't you? Well, definitely faster than Italians, I think)... I was saying, at normal speed or when speaking fast, I think something strange actually happens...