Is it possible to browse by Google eBook topic

Here is the link to the API: https://developers.google.com/books/docs/v1/using#WorkingBookshelves

The URL that you need is:

https://www.googleapis.com/books/v1/user/{userId}/bookshelves/{shelf}/volumes

That will work as long as the shelf is public, otherwise you have to have an API key by putting this GET variable on the end:

?key={yourAPIKey}

You can use a small JavaScript bookmarklet or user script.

A working JavaScript piece of code that I used in the past for multiple sites (adjusted to the current one):

var textArea = document.createElement("textarea");
textArea.style.position = 'fixed';
textArea.style.top = 0;
textArea.style.left = 0;
textArea.style.width = '2em';
textArea.style.height = '2em';
textArea.style.padding = 0;
textArea.style.border = 'none';
textArea.style.outline = 'none';
textArea.style.boxShadow = 'none';
textArea.style.background = 'transparent';
textArea.value = document.location.href.match("https?:\/\/.*?\/books")[0] + "?id=" + document.location.href.match("id=([a-zA-Z0-9]{12})[?&]")[1];
document.body.appendChild(textArea);
textArea.select();

try {
    var successful = document.execCommand('copy');
    var msg = successful ? 'successful' : 'unsuccessful';
    console.log('Copying text command was ' + msg + ': ' + text);
} catch (err) {
    console.log('Oops, unable to copy');
}

document.body.removeChild(textArea);

I just tried it as a booklet and it works great (on Chrome at least).

A bookmarklet ready code (minimized):

javascript:var textArea=document.createElement("textarea");textArea.style.position="fixed",textArea.style.top=0,textArea.style.left=0,textArea.style.width="2em",textArea.style.height="2em",textArea.style.padding=0,textArea.style.border="none",textArea.style.outline="none",textArea.style.boxShadow="none",textArea.style.background="transparent",textArea.value=document.location.href.match("https?://.*?/books")[0]+"?id="+document.location.href.match("id=([a-zA-Z0-9]{12})[?&]")[1],document.body.appendChild(textArea),textArea.select();try{var successful=document.execCommand("copy"),msg=successful?"successful":"unsuccessful";console.log("Copying text command was "+msg+": "+text)}catch(e){console.log("Oops, unable to copy")}document.body.removeChild(textArea);

Side note

It was taken from a user script I created for eBay, to share easily a short and nice link.
If you want, you can look at the full script, which adds a small button to each item page on eBay and adjust it to Google Books.
You can find my script at one of those links: GitHub / OpenUserJS.