Suggestions for creating 3D Bounding Boxes of Books on a Shelf from RGB-D Images
Exploiting the 3D geometry is important here. From the depth image, you could get the 3D point cloud by backprojection. Once the point cloud (set of 3D points) is given, you might benefit from VoteNet:

Deep Hough Voting for 3D Object Detection in Point Clouds, Charles R.
Qi, Or Litany, Kaiming He, Leonidas J. Guibas, ICCV 2019
https://arxiv.org/abs/1904.09664

If, in addition, the color (RGB) information is available, we could utilize within the same framework, this time maybe using ImVoteNet:

ImVoteNet: Boosting 3D Object Detection in Point Clouds with Image
Votes, Charles R. Qi, Xinlei Chen, Or Litany, Leonidas J.
Guibas, CVPR 2020
https://arxiv.org/abs/2001.10692

Both of the papers provide open source implementations. See here for VoteNet and this repository for ImVoteNet. Note that both of these are methods that require heavy supervision, i.e. large amounts of annotated training data. If such data is not available at your disposal, I would start looking into hand-crafted algorithms, e.g. descriptors or point pair feature detectors.
Hope this helps.