Where do I can download free books related to similar Chinese words ( 形近字 字典) ?

There are some books, like《形近字辨误100例》、《常用形近字辨析词典》、《形近字辨似》 、小学生形近字词典。
You can choose which one suitable for you, I will upload it for you.

There was a published dictionary regarding 形近字字典https://baike.baidu.com/item/%E5%BD%A2%E8%BF%91%E5%AD%97%E5%AD%97%E5%85%B8/5788627

But it was a very old dictionary and it would be not easy to find pdf version.

You can try apps：
https://www.pc6.com/iphone/797712.html

or online:
https://3389x.com/similar/

you need a native speaker to help you ,that will enable you to achieve the desired effect quickly,a good teacher can use those words that you learned before to increase more similar words for you easkly,and she will explant it more clear to you ,save you money and time,try me

Hello ,my name is Mia缪 ，and i am an online Chinese teacher of Amazing talker ,I know lots of similar Chinese words, and i often use that to teach my students to help them quickly increase their vocabulary,you can find me there