I have authored a book for Manning (Java testing with Spock) and was approached like you are (they noticed some of my technical articles)
My advice
1) Unless you are going to write a big hit, you are not going to earn match. You write a technical book for prestige, not money.
Writing a technical book for money is the wrong reason to write it. Stop now.
2)You should really know your topic well. I mean REALLY know it.
3)The amount of effort it will actually take will be 6x the effort you think it will take.
4)Make sure that you have enough free time and you have discussed the matter with your significant other (and that she/he approves)
Depends. I find sufficient quantities of money to be pretty motivating as well as quite useful. I don't have any problem with the satisfaction that comes from teaching or with egoboo but it's hard for me to justify the huge amount of work that goes into a book based on those non-monetary things alone.
As has been remarked elsewhere, the monetary rewards don't need to come from royalties. Books are quite valuable as validation for people who make income from other sources--and therefore help support consulting and other income streams.
My advice
1) Unless you are going to write a big hit, you are not going to earn match. You write a technical book for prestige, not money. Writing a technical book for money is the wrong reason to write it. Stop now.
2)You should really know your topic well. I mean REALLY know it.
3)The amount of effort it will actually take will be 6x the effort you think it will take.
4)Make sure that you have enough free time and you have discussed the matter with your significant other (and that she/he approves)