It depends a bit on your objectives - do you want to build your own OS, learn more about the theory and algorithms used in operating systems, or do you want to know details about commercial OSes?

For a hands-on view on building operating systems, I can recommend taking a look at xv6 [1] as the next step. This is a modern reimplementation of a subset of the 6th edition Unix, running on RISC-V in its current version (an older version targeted x86) [2].

The classical textbooks used in OS courses (Tanenbaum's "Modern Operating Systems", Silberschatz' "Operating System Concepts" and Stallings' "Operating Systems") are more on the theoretical side, whereas Tanenbaum's Minix books are more hands-on. A current, somewhat more hands-on, textbook is "Operating Systems: Three Easy Pieces" by Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau. Additional bonus - it's free [3].

For more hands-on resources, there's a lot of material available. osdev.org [4] provides lots of OS development knowledge in wiki form, Stephen Marz' "The Adventures of OS: Making a RISC-V Operating System using Rust" [5] is a hands-on course written as a series of blog posts to build your own OS for RISC-V (but it needs to be updated since RISC-V has changed in some details since the articles were published). I learned about OS theory and practice using Doug Comer's Xinu books [17] and I still think these are very useful.

You can then dig into older Unix(-like) systems, e.g. BSD [6,7] and/or check out more modern approaches such as Microkernels (I can especially recommend L4, starting from Jochen Liedtke's papers [8] and checking out work on seL4 [9]), capability-based systems (CHERI [10] and an old, but good overview book on capability architectures [11]), or - for something a bit more exotic - Plan 9 [12].

If you want to gain an insight into commercial operating systems, there are a number of interesting books on Apple's OS X and iOS [13,14], Windows [15] and even VMS [16].

[1] https://github.com/mit-pdos/xv6-riscv/

[2] Russ Cox, Frans Kaashoek, Robert Morris
xv6: a simple, Unix-like teaching operating system
https://pdos.csail.mit.edu/6.S081/2020/xv6/book-riscv-rev1.pdf

[3] https://pages.cs.wisc.edu/~remzi/OSTEP/

[4] https://wiki.osdev.org/Main_Page

[5] https://osblog.stephenmarz.com

[6] Marshall Kirk McKusick , Keith Bostic, Michael J. Karels
The Design and Implementation of the 4.4BSD Operating System
Addison Wesley, ISBN-13: ‎ 978-0201549799

[7] Marshall Kirk McKusick, George V. Neville-Neil, Robert N.M. Watson
The Design and Implementation of the FreeBSD Operating System
Addison Wesley, ISBN-13: 978-0321968975

[8] https://en.wikipedia.org/wiki/L4_microkernel_family

[9] https://sel4.systems

[10] https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/

[11] Henry M. Levy
Capability-Based Computer Systems
https://homes.cs.washington.edu/~levy/capabook/

[12] Francisco J Ballesteros
Notes on the Plan 9 3rd edition Kernel Source
http://www.r-5.org/files/books/computers/internals/unix/Francisco_Ballesteros-Notes_on_the_Plan_9_Kernel_Source-EN.pdf

[13] Amit Singh
Mac OS X Internals: A Systems Approach
Pearson 2006, ISBN-13: 978-0-321-46795-9

[14] Jonathan Levin
*OS Internals (Volume I: User Mode, Volume II:Kernel Mode, Volume III: Security & Insecurity)
https://newosxbook.com/home.html

[15] https://learn.microsoft.com/en-us/sysinternals/resources/windows-internals

[16] VAX/VMS Internals and Data Structures
http://www.bitsavers.org/pdf/dec/vax/vms/training/EY-00014-DP_VMS_Internals_and_Data_Structures_1984.pdf

[17] https://xinu.cs.purdue.edu