I have no idea if it's already here, but I would recommend He can who thinks he can by Orison Swett Marden.

It's a fairly old book, published around 1908, and written in an old school sort of style, but despite being over 110 years old it is a gem of the old school success/self-improvement style books.

The book is actually a series of motivational essays on various topics, "He can who thinks he can" being the very first essay, it being about not letting anything stand in your way and believing in yourself. Alot like Carnegie's Win Friends and Influence People, it's one of those texts you should come back to every now and then, and I do when I feel discouraged. Rousing much!

I found it on Archive.org whilst searching for out of print books on Smedley-style Public Speaking to aid in Toastmasters. There's quite a few on there along with success/self-improvement stuff.

You can get the ebook here or read online:
https://archive.org/details/hecanwhothinkshe00marduoft