Malaysia, Singapore and Brunei are the most developed countries in Southeast Asia, and tourism is a long-established business in the first two. English is widely spoken and travel is easy. More than anywhere in the region, you could even get by without a guidebook - local tourist offices are pretty good for info.
That said, if you do want a guide, this is probably the most reliable English language one for practical details. The 10th edition, published in January 2007, actually seems to have been more thoroughly updated than some previous ones, with some new attractions and accomodation options added - unlike in certain other LP guides, where only prices are changed from edition to edition. Of course practical details may change even by the time the book is published (remember it was researched in early 2006), but in general these 3 countries are pretty stable and inflation is low. Sure, the odd errorous/outdated info did slip in, but is far outweighed by all the useful stuff.
My only complaint about this book might be that it still pretty much concentrates on established, popular tourist attractions and major cities/towns. Don't expect to find many tips on locating hidden gems or exploring remote corners of the country, particularly Borneo. For that kind of info, I found the Rough Guide to these countries better than LP, though RG's practical info is often more dated. Decide what's more important to you!
If you are going to these countries for the first time, and only have a few weeks on hand to spend there, you will probably find the information provided by this guide both sufficient and largely accurate.