When my son was in primary school it was definitely called CT. Now he's moved up to middle school it's become ICT. So I'd say a CT textbook or ICT textbook. The initials are used as the subject name much as in PE or RE or PSHE
If you really want to stick with the word
computer then I'd go for
a computers textbook - which completely contradicts everything I said above about having to say
computer class because its a compound noun blah blah
I give up
.....