As a professional (not that I'm good but this is one of my main source of income) educational writer and a private tutor for many years, I'm going to share with you parents on buying workbooks from certain publishers for cetain subjects.
1. Most importantly, get the past years questions (soalan-soalan tahun lepas) from Cerdik Publications. The publisher has the rights to publish the real past years exam papers from UPSR to SPM. Go to your nearest bookshops and they will assist you.
2. Bahasa Melayu - Sasbadi books (both KOMSAS, Tatabahasa and Pemahaman) are the most realiable. You can also use Oxford Fajar Bakti.
3. English - Pearson Longman used to be very good but in recent years Penerbit Mega Setia Emas is doing a good job. If you intend to buy English Literature books for your kids, you can look out for Setia Emas' Baby Steps series.
4. Science - Oxford Fajar Bakti is good. Their reference books is highly recommended compare to Sasbadi. Note: becarefully if you are buying Year 5 or Form 5 Science books. Some publisher did not follow the latest syllabus because it was the consequence of a rushed production. Choose your books carefuly.
5. Mathematics - Pelangi is the expert in this subject. So get their Math books. Note: becareful if you are buying Year 5 or Form 5 Math books. Some publishers did not follow the latest syllabus. For example a workbook published by Oxford Fajar Bakti made mistake in their last couple of chapters. Then there's another publisher called B* Publisher. The writer from this publisher couldn't even form proper sentences and the editor did nothing to correct the mistake. I complained to the owner of Bookway, PJ, and I don't see the book the next day. The bookshop owner told me a school (in PJ i think) actually chose B* Publisher's series to be used in 2007.
6. For arts subjects like Sejarah and Geografi, you can consider to buy reference books from Sasbadi or Oxford Fajar Bakti. As for workbooks, you can trust the books published by the above publishers.
7. As fo science subjects like Chemistry, Physics, etc, you can also rely on Oxford Fajar Bakti's reference books. You can also trust the workbooks published by the above publishers. Marshall Cavendish is okay as well.
How come I didn't mention publishers like Preston, Info Didik, etc? Well, it's not because their books are no good but the publishers mentioned above are more reliable.
Hopefully this simple guideline will help you in getting good quality workbooks for you kids and of course have your money well spent.