I just finished translating my first book.
I've noticed that when I reach easier passages that use simple sentence structure, I tend to write fluently in my own style. I'm also not terribly interested in preserving the author's style. When I reread these easier chapters, it sounds like my own writing. I could probably pass it off as my own original work if I wanted.
But when I reach more difficult passages that use technical vocabulary and detailed descriptions, my writing becomes stilted. It no longer reads like my own writing.
Moreover, my translation abilities have improved over the course of this project, being that this is the first time I have translated anything of this magnitude. After I went back to reread earlier chapters, I noticed that some passages weren't translated properly (I could tell just from reading my English translation. I know myself very well. I know what I sound like when I'm making something up. When I saw my "BS voice" in the translation, I knew to go back and retranslate).
Fortunately, because I had marked the original page numbers in my translation, this made it pretty easy to go back and referencing the original.
Once I had finished translating the entire book, it was time for my second draft. (That's what I'm working on now.)
Do NOT just open it up with a word processor and fix words. If you do this, the style will be preserved. The easy-to-translate passages will still be fluid, and the hard-to-translate ones will still be stilted.
Instead, open your first draft in one window and have a blank document in another window. Rewrite the entire book.
I've found that my second draft is, essentially, translating Bad English into Good English. But this time, make sure to "translate" it paragraph by paragraph, not sentence by sentence. Note that I have a lot of liberties in my translation.
For some very technical chapters, I need to study the original in depth and take notes. Then, I put everything away and just write from my notes. I don't care about sentences and paragraphs. I just care about conveying the content. Then, once I've finished, I compare it to the original to double check.