As I've said (but keeping in mind that I have taught from these books, but not written or published them, so I can't be sure) the primary purpose of those problems is not for the self-learner, who can mostly ignore them, but for the teacher. I have often assigned a set of odd problems (not all of them, which would be an unnecessary burden for a good student) to be done to learn from (students who need more are encouraged to do more), and then a few even problems to be turned in and graded, as a reality check -- in effect, a take-home quiz. Too many students can think they have learned the material if they can eventually solve everything with the help of the answers in the back, but discover (if they even notice) that they can't do it without that crutch. Self-learners can use odd problems the same way, by not looking in the back until you would be ready to turn them in if you had a teacher. You can get your own reality check.
You could use the even problems the same way: After you think you've mastered a section, try some of the harder even problems to see if you are confident that you got it right (including checking your answers, an important skill that many students don't bother with because they can look in the back or have the computer check). If you aren't confident, go back and learn that material by rereading and doing more odd problems (which might include redoing those you had already done).
You could also ask here about those even problems you aren't sure of. (Don't bother asking about the odd ones, or the even ones you've checked and got right.)
And you generally don't miss anything by skipping the even problems; many books have problems in pairs, an odd and an even at the same level of difficulty, for the teacher to choose. (Some books don't, perhaps more often at higher levels. I have more than once found that I couldn't find an odd problem to teach a special case that is seen in a couple even problems.)
The important thing, as has been said, is to do as many as you need to feel confident that you could do the rest. That's entirely up to you, and entirely dependent on your own skills and weaknesses. You have to time to master the subject, if you work at it intelligently. Too many students in a class don't have (or take) that time, and don't work at it with that goal in mind, but just do the minimum they are required to do.