The SH is /ʃ/ and the CH is /tʃ/. Although they look similar in IPA (both have ʃ), native English speakers never confuse them or even consider them to be confusable. CH is analyzed as a completely different phoneme, not as SH with a T in front of it. Perhaps your book has some mistakes. Note that in initial position, the SH may (perhaps) be rounded somewhat to distinguish it more from the CH sound: sheep versus cheap. Note that in syllable final position, the CH may (optionally) be pre-glottalized in order to distinguish it more from the SH sound: wish versus witch.

Native speakers have no problem distinguishing phonemes [tʃ] and [ʃ].

There are a few words with irregular pronunciation. The letters < ch > are usually pronounced [tʃ] but not always. In < machine >, the < ch > is pronounced like < sh > [ʃ], and in < chorus >, the < ch > is pronounced like < k > [k].

It’s a pleasure, Benny! Keep up the good work, and keep asking the interesting questions.

Yes, the two words "sheep" and "cheap" sound different to native English speakers, and I'm going to see if I can prove it. I'm going to turn on the voice dictation feature on my computer. I'm going to speak individual, isolated words so that the program can't get any cues from context.

Here's what I'm going to say. We will see what the program hears.

Sheep. Cheap. Cheap. Sheep. Ship. Ship. Chip. Chip. Witch. Wish. Witch. Wish. Mash. Mash. Match. Match.

OK, here is exactly how my computer transcribed it. My apologies for the rude word.

Sheep cheap cheap sheep ship shit chip chip which wish which wish Mitch mesh Match Match

The program correctly identified the <em>sh</em> or <em>ch</em> sound fifteen times out of sixteen. It only made one mistake, hearing <em>Mitch</em> (with a <em>ch) </em>when I said <em>mash </em>(with an <em>sh)</em>.

Incidentally, it isn't really a mistake that the program transcribed "witch" as "which." The pronunciation of "witch" is identical to the most common pronunciation of "which." Some very careful (or old-fashioned) speakers will aspirate the first "h" in "which"--it actually is more like "hwich" than "which." There are many bad puns playing on the fact that the two words can sound identical.

My pleasure, Benny. Actually, it occurs to me that those two sounds could be confused in one particular phonetic environment — immediately after N. Offhand, I’m not aware of any relevant minimal pairs. The same is true of /s/ and /ts/ as well as /z/ and /dz/ - the difference is neutralized after N: “patience” and “patients” are pronounced the same. There is a pun about a doctor who is losing his patience / patients. In practice, such word pairs are very infrequent, and there's never any confusion.