你要从字面上说不对,就是to make yourself a part of it和前面的made it a part of yourself不一致.
但我觉得也没错,因为不影响理解就好.
如果你非要从语法去分析：
既然前面说了把这本书变成你的一部分不就是意思说,要把自己和书融合在一起,书中有我,我中有书.而且我在书里写字,在这种情况下,我不也成了书中的一本分吗?
and表并列,如果后面也跟前面完全一样的话,他完全可以用“ and the best way to achieve this”代替.
when you have made it a part of yourself,and the best way to make it a part of yourself is by writing in it.语义也没错.但是 make it a part of yourself 出现2次,靠的比较紧,显得有些重复了.不如：
when you have made it a part of yourself,and the best way to achieve this is by writing in it.
对于原文,我认为也可以说没什么错的,还算通顺,不影响理解.
我觉得这样翻好一些理
Full ownership comes when you have made it a part of yourself,the best way to 【make yourself】 a part of it is by writing in it.
【把你自己融入书中】的最好方法是在书中写字.
and前后不同的说法其实表达的是一个意思,都是书与人融为一体,既然融为一体了,就不用管谁是谁的一部分.
我觉得这样写的好处是,避免了and前后都用make it a part of yourself的重复,或者这样写也可以：when you have made it a part of yourself,and the best way to achieve this is by writing in it.
两种都通顺,外国人更注重读起来通顺（语感）,而不是每一处语法.还是那句话,不影响理解就没错,不用太计较语法.当你读的多了,自然就不觉得别扭了,而且你语感强了,可以忽视语法了,哪句更通顺可以脱口而出了.
希望你能看看那本书,跟传统的学英语方法很有区别,祝你更上一层楼~!