I’ve done the splits and could do a full back bridge to walkover, have put my head on my knees and my hands around my heels in a seated toe touch. When I was less active I couldn’t even touch my toes at all and was nowhere near doing the splits.
The number one thing I can possibly say is that frequency is the most important. I found that I have to stretch an absolute minimum of 4 days a week if I want to make progress in my flexibility.
I trained Tae Kwon Do for 3 years, which is known for it’s high kicks and we used plenty of dynamic, static and contract-relax stretches which I think are all useful in adding to your flexibility.
If I had to pick a single approach, I would use static stretching, but to do the splits I had to incorporate PNF stretches. All forms of stretching have their limitations.
I cannot recommend any books as I’ve never found any that I thought were that great, well aside from an old Health for Life publication (maybe Super Stretching was the title?). I used to own Pavel’s stretching books, Tom Kurz’s book, Bob Anderson’s book, the original PNF manual, multiple books on Thai Massage (which is a form of assisted stretching), and have looked at others aimed at dancers, general athletic stretching, active isolated stretching, etc. There are some good stretches in all of them, but I don’t really care for the approach in any of them.
My goals have always been to develop extreme ranges of flexibility for martial arts (Brazilian Jiu-Jitsu as well as Tae Kwon Do), so I’m biased towards methods that work for those purposes. You may be looking for other goals, so other approaches may be better for you.